OK, I'm old enough to have seen a few friends go through this sort of thing. Basically in any love triangle, 99.9999...% of the time, the cheated on spouse is made totally miserable, the OW/OM has a pretty miserable time of it too, while the philanderer lives the life of Reilly and has a high old time. If he's prepared to do that, he'll be a total gobshite. So far, so much in agreement with most upthread (though possibly, having seen close friends emotionally destroyed by being lured into being the OW, and on occasion , the OM, I have a bit less of a tendency to scream "attach the scarlet A and burn the bitch" than some).
However, another thing not mentioned so far - you've said you've come out of a shitty and abusive relationship. Are you sure there isn't an element of one or both of the following thought processes going on? "All relationships are shit, even if they put on a good face to the world, because mine was, so I'm just helping myself to cookies in a biscuit barrel that frankly are already broken so no one should mind?" (This was the attitude of one woman I knew who'd divorced, then chased a married man, and was left looking a complete plonker when he would have none of it). Or alternatively, part of you thinks (not that this is actually true, I just know that abusive relationships totally destroy people's self worth) "I was in a rubbish relationship because I am a bit rubbish and the best I deserve is someone else's leftovers, and huge physical passion is all I can get, because the emotional connection and fidelity that can and should go with that is only for people better than me."
I'd back well off this situation, maybe try to find a way of being content in your own company for a bit, and if necessary, find a single friend with benefits if celibacy doesn't do it for you while you get your head together (making sure that both of you are happy for it to be a purely physical thing).