I been with him 9 years! and I find him disloyal!

Like to get another girl intimately? Is he Married? If not yeah he most likely would. But does he is the question. But that's all irrelevant. when you treat people like that you get what you are looking for.

Since you've been encouraging him to talk to girls may be he is just trying it out.
I think you should talk to him about it (usually with the 4 most annoying words to guys "we need to talk")

You are irritated that he is "disloyal" because he talked to a "stranger". How would he feel about your dishonesty? Women think it is their right and their job to test men. would you enjoy him doing these kind of things to you? if you are just playing games with him You should leave it. If you have feelings he is being cheating on you. You should leave him. And I don't say that to mean he is most likely cheating. I mean it in the way that if you are having unfounded suspicions about someone you claim to love. It will only blow up in your face. And you will do more damage to another human being than needed. So that is Why I say walk away. But. If you are just bored and looking to just add some excitement to your life. I say grow the hell up. And stop acting like a 12 year old.

"should I test him or just leave it"

I don't know about your boyfriend, but personally, I won't tolerate "tests". If I found out a girl's given me one (regardless of whether I "passed" or not), we're done. Permanently.

Only test if your ready for the unexpected, because if you find gaps in the trust you have for him, then there's no real relationship until those gaps close, and this could be just some harmless fun, but if you go deeper, you could end up damaging something that doesn't need fixing, so think carefully, this could be innocent fun and he could think its just a wind up, but if you go deeper and try and make say things you don't agree with, this could really end all what you already have, if you trust him, let that be enough, if you dont, then question your relationship and work on it, don't go out to destroy it just to find something out, or not,x

That's called entrapment.

I think you should calm down. When I get a text message from someone I don't know, I also reply, and sometime I enjoy exchanging a few messages. It's adventurous and fun and doesn't mean I would cheat on you.
We all like to have some excitement in our lives sometimes.

So you have been with him since you were 11 or so. There seem to be many contradictions in this story as well. Are you controlling and pretend to not be?

No, play roles with him, tell him you're eager to meet him but too shy to talk face to face. Arrange to meet him somewhere. Then stand him up and watch his reaction.

If you have a friend willing to play along, someone he wouldn't recognize, it would be fun to have her meet him for a date and see what he says to her!

Okay so there are plenty of red flags going off in my head right now about the validity of this article. How have you two been together nine years and you are younger than 24? How is that it took him 8 years to tell you he loves you? This is just highly illogical. So for the rest of this I will pretend that you meant months.

So first off it depends on what you mean by "cozy" if you mean he was just texting her a lot about who she is then you might be over reacting a bit, if you mean he's texting things like "I'd love to meet you" or "you sound cute" or as such then that should be major foretelling of the rest of your relationship. If anything you should CALMLY talk to him about what boundaries do you two believe should be in place in your relationship. As much as I hate tests, if after that and he is still texting your cousin you may want to ask her to step it up a notch. Again only after you have discussed boundaries with him. If he does get very flirtatious with her (by very I mean asking for her picture or wanting to meet) then you may want to call it quits as this is part of his personality that he needs to grow out of.
Trust me, you don't want to get to the point of being engaged or married and your heart gets destroyed because he goes too far with this kind of stuff and then cheats on you. Sorry, that may of course be my personal bias

I once had a girl I was dating "testing" me exactly like that, one time, within days of meeting she called me REALLY drunk from her half sisters cellphone, now said sister was from a different state and practically the only person from that state I have ever met, so months later I get those test messages from a number of that state, I inmediately know "it is her" (she never even mentioned she had a half sister, weeks later I asked her were she was she said "at grandma's" only later admitted that indeed she was not getting wasted with her grandma :P I guess the alcohol was that much that she didn't even remembered calling that night).

my Girlfriend knows I am social and that I have many good female friends, but this other girl I was dating was like "hell no, he can not talk to anybody" so she tested me to see if I would bite the hook, so I get those messages from her "grandma" telling me how handsome I am and I am like "ohhh thanks, I am blushing" and a lot of smiley faces and winks, I was not being "disloyal", I KNEW it was her, and quite honestly it pissed me off, but took it easy and decided to be playful about it, I never truly flirted with the "grandma" just wrote " :) you seem to be charming too", and when she was like "come home and watch a movie with me" I simply said "haha thanks again but from the phone numbre I can see you are far away" then she was like "yes, the phone is from away but I am visiting here" and I am like "so :) enjoy your stay" and stop answering for good.

next thing I know she is complaining and everything and I am like "I am the one who should be angry, if you trust me fine, if you don't gom play games on another man", and well, she did, she found herself a boytoy and I wish them look, even if she had decided to stay, before the incident I had already gotten creepy vibes from her, the "test" just proved me right, she came back when her anger faded but I knew better, offer her my friendship, and it was honest, it still is, but I don't want a girl like that for a relationship.

I understand, people can be hurt and then find it hard to trust again, but that is your problem and only you can solve it, if you have trust issues even if he was the sweetest man you will be second guessing everything and angry and sad, is not up to him to solve your trust issues, he can hyelp, and he would probably be pleased to, and you may say "and if he indeed is not worth my trust?" well, again, love requires faith, people will fail intentionally or not, if he is someone you try and trust do it, if not, don't test him, leave him, you are irritated by him "failing" your "test" think how irritated you would be if he were testing YOU, I bet you will be "my Boyfriend doesn't trust me, then he SURELY doesn't love me at all, if he loved me he would believe in me"

what was the text saying . if it was like who is this how do I know you. that be fine.. if he is saying lets hook up or share pictures, I wouldn't be with the guy at all. it would say to me that he's always looking for someone better.

He's going to have sex with your cousin, simple as that. Her sweat will be on the pillow you sleep on.

Just leave it. He was talking to a stranger. It's not like he was sexting somebody. Christ woman, get a grip on yourself.

He is doing exactly what you told him to to. You did not go into detail about "how the conversation went", meaning if it went sexual or not. Or if they talked about hooking up or anything. Or if she asked if he had a girlfriend and he lied or something like that.
But it sounds like he is going exactly what you told him he "should do".
Don't be one of those, "don't do as I ask" females. Some guys do listen.

Don't get too hot-headed over this because you could end up overreacting. If the convo gets iffy though, make it known to him.