I have been together with my girlfriend for about 6 months now. We get on great and our sex life is perfect. We see each other a lot and she always stays over at my house. She lives with her mum and brother, he is a couple of years older than me. I went round at the weekend, as I do most weekends, to see them. Just her brother was in as my girlfriend and her mum had gone into town. He said they wouldn't be too long. Me and her brother sat and watched some TV whilst chatting about various things. Next thing I know he's making a move on me. We shared an extremely intimate kiss and ended up going upstairs to bed and having sex. It was great and felt right but afterwards I felt so disgusted with myself. Neither of us have said anything to anyone but everytime I go round to their house there's a certain awkwardness but me and him smile at each other and sometimes when no ones around share a kiss. Me and my girlfriend are still sexually active and it still feels brilliant, what do I do?