At the start you are only dating, you are not in a relationship, hell, you just met Lena in a local coffee shop and barely know her, and she barely knows Ian, so they're not even remotely close to being a couple. They are both dating different people, playing the field, and the story revolves around their lives and how they intertwine. Eventually, you can become a couple, Ian and Lena can as well as other combinations like Ian and Cindy, Ian and Allison as well as different combinations with Lena, all depending on your choices. Once you're a couple, then sleeping around becomes more of an issue. i find it difficult to believe you haven't had a relationship option yet, I had it several times, with Lena, with Cindy (which is a really odd one) etc.
About the only problem I honestly had was when you decide for Ian to sleep with Holly, Lena gets all upset and won't do anything more with you even though you were not a couple and she encouraged Holly to go for you and it was Holly's idea to sleep with you even though she knew you were seeing Lena... but somehow it's okay, it wasn't Holly's fault, but you're some sort of jerk. That was piss poor writing. Especially given the guys Lena has/can sleep with. I was like... "WHAT'S THIS BULLSHIT?!".
OTHERWISE, you can choose not to sleep with anyone, or stick to only one person. I played through it again and the next time I avoided any relationship with Holly as that line is just too messed up in the writing.
Another messed up moment was when Lena gives you a shocked look when she finds you at Axel's with Cindy, but then when you play her, you have the option to sleep with Axel... LMAO...
I guess we just have to remember this is FICTION and the writing is imperfect. Personally, I preferred just a single character like GGGB with more options for them than playing two characters. I think the relationship writing wouldn't be as difficult.