Well, this is a very difficult answer. I'm sorry for the wall of text:
Romantic attraction can be completely platonic in nature. There can be no trace of sexuality in it. A gay guy can feel it for a straight girl. It can be anything from spiritual, intellectual, emotional. Of course it can encompass all of them, and also be sexual. But they are not necessarily inclusive, and they are not mutually exclusive.
At the same time, sexual attraction can be purely physical. It's my opionion, of course, but I don't believe they have to overlap. I believe someone can have a sexual relationship with somebody without that marring or hurting an emotional relationship they have with someone else. I don't believe it's cheating, as long as everything is talked about from the beginning and they go through the basic notions of what it's to be expected of each other.
Of course, if there isn't an explicit agreement, and you hide from your SO that you're having sexual relationships, even if you consider them to be meaningless, then it'd be wrong, it'd be cheating. I believe in honesty and full disclosure. Everything else is second to that.