As people have said, you can't expect someone to stay in a sexless relationship; if sex is an important factor for them it's perfectly acceptable.
That said, sex is awesome. It feels good and makes people happy. And happy people are better to be in a relationship with.
I generally base the decision to sleep with someone on whether I'm comfortable with them, attracted to them, and respected by them. Comfortable, because I'm not a fan of awkward sex; attracted, because that's the whole point; and respected, because I'm fucking amazing and I deserve respect. Not on love, not on an arbitrary number.