If you're not looking to marry the girl, what's the problem?? It's your mindset, that's the problem.
Clearly, you're not marriage material to her, so she's fine with it and you should be too unless your intentions for her future are different?
Before I got hitched, three of the last four girls I saw, in some variance, were into something or someone or some group thing. Who gives a crap?
In one of my more 'surreal' moments(I guess, but I was none-to-bothered) a girl that was into coming over to see me was getting plowed on the floor in front of me, whilst I watched the telly!! A pal, that came over just took advantage cos she was in her see through negligee(something she popped on when she stayed for the weekend as it was 'my flat, my rule'!!). She knew I wasn't the jealous type, wasn't gonna marry her and sex was just sex to me.
That was over 10 years ago, like, but hey! sh*t happened and it's all true!
*sigh!* Ah, the memories!!
I kinda miss my old life, but I guess the debauchery was getting out of hand...