Advice please

I am going to marry my partner in August of this year. 
He has two children, a girl aged 18, and a boy aged 16. When I met him, he lived with his parents. 
He is in construction and is progressing within his career. He has approx 10k worth of credit card debt. He doesn't earn a huge amount but he has ambition and has already demonstrated that to me. 
I have no children, own a home as well as a relatively successful business. 
Half my mans wages go to his children, and he gives me £75 per week towards the upkeep of my home. He has been living with me for over a year now. 
I am 36 and would like to have a baby at some point in the not so distant future. 
We recently had a massive argument, which resulted in him packing all his stuff and going back to his parents for six days. He's back now, but I just don't feel the same any more. 
Basically, his eldest daughter is rude and not very nice. I cook for them both every Wednseday. She barely speaks to me, shows no gratitude or respect and quite frankly, I can't be bothered with it any more. One evening, my partner came home after a few drinks and told me that his son may have the opportunity to go over seas with football. I got so excited and said that would be an amazing opportunity. He didn't agree saying he wouldn't want his son to go off alone. I respectfully disagreed, anyway it resulted in him laughing in my face saying 'what do u know about being a parent, you know nothing!' It really hurt me. So unfortunately, I did something stupid and said 'how can u tell me anything about parenting when you have raised that as a daughter!' I said she is rude, lazy, spoiled, ungrateful and has a bad attitude.' This did not go down well at all. Then the following day I said I feel like I'm subsidising his kids, as if it wasn't for me enabling him to pay £75 per week here, he wouldn't be able to give his kids as much money. That's when he left. 
He's back now and I was hoping we would get stronger and closer together. I still feel so upset he involved his parents as I'm such a private person. 
I have a pre-nup which he is happy to sign but he's concerned that if we had a baby, that's potentially if I left him, he'd be back to square one with nothing. 
What do u guys think? 
I am still hurt by him leaving. Does this show me he has no stick ability? I'm so confused right now. I think I've rushed into this a bit.