I agree - this is awful!
Until you know a relationship has a decent chance of lasting (eg you have been "together" for at least a year), then children shouldn't be meeting either parent's "special friend" - and they certainly shouldn't be moved in and families joined together.
I've seen people of both sexes moving in with the latest love and then 6 months later it's all change and there's another one. And the number of women who immediately have another baby with their latest squeeze* and the bloke is out the door before the baby's arrived.
If you fancy a shag - have a shag, but for crying out loud, put your children first and give them some stability. A lot of men and women don't really want to take on somebody else's kids anyway - and the children are made to feel in the way. (Or worse - too many men target vulnerable women with children for perfidious reasons)
*"Latest squeeze" sounds very old-fashioned. Is it old-fashioned?