I found myself in a similar situation once upon a time. I did love the guy hence why I carried on, even though I felt stupid and angry half the time. After several months I finally broke it off cos it was doing me more harm than good.
I think that's the most sensible thing I've ever done.
Looking back on it I feel somewhat embarrassed by my clouded judgement but I also know that no matter what I would always have acted that way cos at the time it felt right. He and I hadn't started dating though we fooled around a bit [sex wasn't involved cos I'm not sexually active] when he started seeing another girl. We left it for a bit and then old habits resurfaced and I couldn't say no. I'd ask him about her and he'd just say, "I want you but I can't do it to her, she likes me too much and I care about her... when she wants to break it off then I will."
Tbh I think this kind of behaviour shows a lack of caring for either side. He shouldn't be with either of you cos he obviously isn't ready to commit to either of you, but until you decide to do something - cos he won't, and the other chick is unaware - it's just gonna carry on in the same way.
Obviously there is no way to generalise people and feelings, but imo from my own personal experience he needs to shape up, but won't. He's getting the best of both worlds so what motivation does he have to change? I don't like to give prescriptive advice of this kind to people I don't know personally but frankly, I think you should gather your strength and block him out. It took me a while and when I did, I couldn't face him cos I knew it'd just become the same. I didn't see or talk to him at all for about 4months, and it was 7months before I saw him without having someone else in the room - even that only happened once. I still don't feel totally comfortable around him.
But go for it! Find someone who isn't messing both you and his significant other around. In the long run this guy won't be worth it. And if he truly is, that'll work itself out in its own time - in the meantime, look after you.