I don't think YABU at all, but I also don't think it does any good to try not to understand why some parents behave like this or to hold onto anger or hurt because of it.
My Mum and Dad split when I was 4. Nasty custody battle where it transpired my Mum had had an affair, something in those days that was thought of as horrendous. My Dad got custody, but I am not sure he ever wanted it, his parents had pushed for it, IMO. My Dad then remarried to my vile stepmother who was nasty, bullying and violent towards my sister and I. (She would tell us to tidy our bedrooms and then untidy them 5 mins or so before my Dad came home from work so it looked like we hadn't tidied them at all and we'd get smacked for it). Eventually she told my Dad it was her or us, so we were packed off to live with my Mum and Stepdad. My stepdad then abused me (sexually) and my Mum still stayed with him and I went to live with my friend and her parents, who fostered me.
I have no idea why my Dad chose my stepmum, neither do I care. I don't see him and other than at my sisters wedding in 1999 I haven't seen him for 20 years. I am happy with that arrangement, he means nothing to me. I realise that due to a difficult childhood herself my Mum has had real problems with depression and self esteem her whole life. She eventually left my stepdad after it transpired he had also abused my sister and then went on to have a string of unsuitable boyfriends. It is only in the last couple of years she has actually realised she can cope without a man. I feel very sorry for her. I knwo she deeply regrets how she behaved and she does love us, she just never knew how to be a Mum. She still doesn't really, but she does do her best and I no longer feel any anger at how my childhood turned out because of her.
I always try and think of the positives and one thing I have learnt is to make absolutely sure my children feel safe and loved. I am not with their father, but have a good relationship with him and my now husband loves my children like his own. I am not the perfect parent, but my children mean everything to me.