I am a father to two boys of 18 (okay, legally an adult !) and one of 12. I've been in a 7 year relationship with a woman who also has two kids. A girl of 17 and boy of 15. I have my boys 3 days a week and she has here's all the time. The relationship worked by me going over to her place when my boys were at their mum's and we'd either stay in or go out which we did quite a bit over the years when it suited her.
Her kids would occasionally go to their dad's place but he was in time honoured fashion very unreliable and often didn't do what he said when agreed or would get into massive arguements with the kids.
Basically, I led two lives, one with her and her kids and one with my boys. I was very much around her kids for years and although they accepted me in the end, the daughter in particular didn't want to have much to do with me. My partner would spend occational time with me and my boys but due to a busy life and finding my younger son too noisy and demanding she kept herself away as much as possible.
She let me know on a regular basis what she thought of my son and that she couldn't bear to be around his noise and demands. I swallowed this with sadness but kept the relationship going because I was good at seperating the sadness from my emotions and I loved her.
The final straw was her asking me to give over one evening every week without fail to us going out together. It just happened that this is a day of the week that is the most important with my younger son and she said that this was the best day of the week to suit her needs. This night out every week was non negotiable and if I didn't do it, our relationship was over. I think there is an element of feeling she put her kids second to our relationship over the years and they suffered the consequences of her not being around. An important point to add is that I never made any demands that we go out and leave her kids alone but she did it for us. If anyone can be bothered by my story and has a moment to comment, I'd be glad to hear honest responses, especially from parents who have been in similar step family settings.
I've had a fair bit of talking with male friends but want to particularly get mother's responses.