Girlfriend made a new guy friend - how should I respond?

Anonymous
I'm going to try to fully explain this so that there's a good idea of where everyone stands .

I've been dating my current girlfriend for almost five years . We're pretty far along in our relationship: we're best friends, we've discussed marriage and theorized about married life . However, from time to time, we've questioned the validity of our relationship because ours is the only mature relationship we've ever had and we lack dating experience with other people . Usually, I'm the one who has had this sort of wanderlust, since my girlfriend has dated and had experiences with other people in the past . But in every case where I had an opportunity to date someone else, I decided that my current relationship wasn't worth sacrificing .

We'd both been attending the same college for a while when I graduated last year . She took off the fall semester to work and build up some funds for this spring . Now she's back at college, but this time she's alone . Near the beginning of the semester, she quickly became friends with a guy in her major who shares a lot of her classes . Once she found out that he liked her, she developed a reciprocal crush on him . She found that he was intelligent, and he could discuss things about her classes that I couldn't . She said that since I wasn't down at college, she felt the need to substitute me with someone else, even if he wasn't as good, and she implied that between the two us, we'd make an ideal boyfriend . When I made it clear that this wasn't okay, she agreed .

However, she replaced the desire to date him with the desire to "fool around" with him sexually . The classes that they attend together deal a lot with sexuality, and apparently this casual discussion bled into their friendship . They've talked about sexual matters, he has made jokes about fooling around, and they've both told each other that they have seriously entertained the idea of having sex with each other, but neither wanted to make it happen due to the potential to ruin my relationship with her .

From what I know about him, he doesn't seem like a very good person, but my girlfriend seems to like him . She's made it clear that she's not interested in dating him or messing around now, but she wants to remain friends . She's joined his circle, and she accompanies them to go out drinking, watch them smoke hookah, etc . (things that we have never done together) .

I want her to have freedom, but the undercurrents of her relationships with he and his group of friends have me a little concerned .

What's okay, what isn't okay; how am I supposed to feel ?
Girlfriend made a new guy friend - how should I respond?
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