Not in itself, but it can be suspicious. Why be on a dating site or app when you're in a relationship? Maybe it's for an ego boost and validation from other people, or maybe you're using it to keep your options open or hook up behind your partner's back. Doesn't sound good, especially if you're hiding it.
Maybe if you trust each other enough, and you both know it's just for fun and not hooking up (or you're in an open relationship and that's okay), it's not a big deal, though. Each couple has to decide for themselves where the line is and what they're comfortable with.
No, but I don't think anyone would be okay with it. I would not want my SO to be on it. But I am single and do not take Tinder seriously, I have it and I go through it but I don't think I would ever actually meet someone on it, it is just fun to be completely superficial and go through pics. But, even with that being said, I would delete the app and hope my SO doesn't use it, I wouldn't trust it.
Well, when I was in one, I deleted the app off my phone. I think people that have it still on theirs... well, I think there's something a little off with that. If you're in a relationship, there should be no reason to have it on your phone. It's not cheating, but it can definitely lead to that.
Strictly speaking, it's not 'Cheating', but really, don't be surprised if you get dumped over it! Just make sure your partner is ok with it, and don't point out the hot people on Tinder or you'll make them even more insecure.
I'm currently talking and seeing a girl I met on there yet I have not deleted the thing. But neither have I used it. Took it off from my main screen and now it's somewhere on my phone. I have no intentions of using it now but in hindsight it would be easier to let it remain there. I mean heck, if I were to officially get together with her or anyone, keeping it around to then play games and compare who could get more matches would actually be fun lol