View from the other side here.
I'm an 'ow' for now, unintentionally, I met a guy and we got on well and things mushroomed.
I've always been aware he's married and no it didn't stop me. It almost stopped me, I took a few days out early on to really think about things, and eventually decided to continue with what had so far developed, on the grounds that I'd rather have the experience of him for however long it lasted, than not have it at all.
I know nothing about his wife and don't ask. I wouldn't presume to ever be able to understand the intricacies of their relationship and why he is dallying about with me. I'm also under no illusions about where this is going (nowhere) and when he'll leave his wife for me (fifth of never).
I've never done this before and in the future would stop myself before I got in too deep. But I do love this man - this lying, cheating, adulterer. It's not logical but I think it's because we only have half a relationship, I only need to love half of him. Which is the bit that spends one or two days a week with me and is a really great guy during that time.
As for blame: I'm sure his wife would want to scratch my eyes out if we ever met. Day by day however I don't think much about her. He doesn't 'slag her off' to me in fact he talks about her with obvious regard, there is not even much wrong in their marriage, from what I can make out (though as I said we do not discuss it). He doesn't make promises to me, I do not like 'drama' as you put it, steer well clear of any risk, and in fact I have only told one close friend about who I'm seeing and we don't chat about it much. I have no bad feelings towards his wife and don't want to 'get one over on her' in fact I'd rather I didn't, he's got a great family, great house and job, I'd hate to be the one to 'help' him lose all that. And no, I don't want to move in with him and take over where she left off. I've got one dc and we've got our own little life ticking over miles away from where he and his family are and I'm glad to keep that separate.
Also, I'm well aware that he and I have little in common beyond the romance that's keeping us together for now. He and his wife have a long marriage and from the little I know about them both, seem well enough suited and contented enough for that to endure til the end.
If this was a single guy we'd have had a shorter relationship which burned out sooner on the grounds that we have few shared interests. As it is, because we don't see each other much and because we are each feeding the other's need for romantic attention, I guess, the whole thing is being strung out over a longer period of time. Quite a lot of the attraction is having someone with whom one can laugh and forget real-life troubles. Don't underestimate the lure of that to people who don't already have it.
Also don't assume that the OW is always a plotting hyper-sexual seductress who wants your life for herself. Sometimes she's just fallen for the wrong guy, however stupid and unethical that might be, and has decided to be selfish and wallow in the small joys he brings into her life for a while. I know our 'thing' is going to end and I'll be sad (for myself) when it does, but I know I'll be able to look back on it as an intense and passionate few months with a man who made me feel special and loved, and that feeling is quite honestly enough for me to shelve my own sense of sisterly solidarity.