I have a girl that has a boyfriend. I have known this girl for awhile, and she has clear attraction for me. She isn't the role model girlfriend...she has kissed other guys while she has been with her boyfriend of 2 years. She hasn't fucked them. Anyway, we worked out one day together and I took her back to my dorm. I asked her to go get some food with me but told her that I wanted to take a shower first. At first she was going to go home because she didn't have clothes on but I convinced her to take a shower anyway after a little more persistence.

At first, I did really want to take a shower lol, but honestly, the tension in that shower, two naked people can't just shower. It makes no logical sense. So after we both took our little showers, I escalated and started fingering her. Her anti-slut came in, not surprising. She said that she can't do this...then she said she hasn't fucked two guys at once before, then she said she isn't ready to do this now, and other things. I battled with her objections for a bit but in the end, I didn't bang her. I don't know how to proceed with this girl. Should I keep having shower sessions and hope for the best? I think she does like/love her boyfriend, but she has made out with guys so I always question her logic.

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Stay hungry, stay foolish- Jobs

This won't be a popular answer but I think you should just chill out, be her friend and not try to bang her. You really have to ask yourself if getting laid with that one girl is worth messing around with her relationship and perhaps her well-being (if it leads to a breakup). Now on one hand I realize that it is her boyfriend's job to keep her happy and it's their job to make their relationship work and to stay monogamous (if that's what they want).
On the other hand, it might be best to just back off and fuck some other girl and not get your hands dirty on this one. Maybe they will break up and then you can dive in. Or maybe you can just stay friends. Just make sure you stay true to your values and know what your motivation are. For example - you might want to fuck her just because you really like her and are really attracted to her. Or perhaps you want to just have sex with a girl that has a boyfriend for the ego-boost.
Know thyself

I am speaking from experience take it for what it's worth.

Ask yourself these questions:
- if you want her to be your girlfriend in the future, just be aware that she will likely do to you what she is doing to her boyfriend right now. Are you going to be ok with that ?

- if you just want to nail her because shes a hot woman and you are a man then go ahead. But what if in the future you will have a serious girlfriend and you will know from experience what little it matters for a girl to be in a relationship and fuck around ?

- if you just want ego boost, because it's a "challenge" to fuck someone with a boyfriend because it proves that you are better than that guy, then ask yourself why do you need this kind of validation to feel good about yourself ?

Not saying don't do it. But if it was me I would not do it personally.

Sorry guys, I have to disagree.
It's her boyfriend's job to keep her protected and sexually satisfied. He's definitely not doing that and she doesn't deserve him. It's her own decision whether to sleep with other guys or not if she's not getting any satisfaction from her boyfriend.

I somehow sense their relationship won't last long. If it did, there's going to be at least one huge breakup/fight to solve it. At least that's what I've seen a couple of times before.

If you really like the girl, shower with her and don't argue, just escalate. Arguing turns her off immediately.

I didn't say he shouldn't do it, but just to consider a few things. I think that especially when he likes the girl is when he should be more careful. If he doesn't give a shit about her but he just wants to fuck because he is horny then I agree with you.

Answering your questions...

I don't want her to be my girlfriend at any point.

- I don't need an ego boost, she is hot, so I just want to bang her brains out.

-I already know from experience what it little it matters for a girl to be in a relationship. Not all of them are like that, but once you are in the game for awhile, you learn new things about human nature. You will experience these things too at some point. Not all the girls you game will be single, at some point a girl will hide her bf from you until she calls it quits and then reveal that information.

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Stay hungry, stay foolish- Jobs