They didn't resent the children being in the house, nor paying child maintenence, what they don't like is being kicked out for no good reason.
I'm actually quite surprised that so many of you are taking the woman's side, Im a good judge of men and they did not deserve to be treated that way.
On a happier note my youngest daughter had the opposite problem, she was the prettiest of my 3 and a lovely temperament. Started dating her husband at 18, he is tall, dark, very good looking ( I fancied him myself) and there is a lot of money in the family, the perfect SIL.
The problem was he refused to marry her, she stuck it out for 6 yrs then walked out and went backpacking, then after 3 months went back to him. Still no marriage, so she got pregnant, son born, no marriage, pregnant again, then at about 6 months gone I got a phone call one Monday, " mum do you want to come to a wedding on Friday". That was it both sets of parents, two witnesses and a pub lunch afterwards.
His mother had put her foot down and told him to get married, they now have 4 children, MIL is best friend and helps a lot with the children. It all worked out in the end despite the frustrations