New Stepmum Help

His 15 year old daughter has asked if her 19 year old boyfriend can stay over. 

My partner doesn't have a problem with it as he thinks the guy is nice, a good influence and he would like to get to know him better. 

I'm completely uncomfortable and have pointed out its illegal as she's underage and they are more than two years gap; there is a 13 and 9 year old living here that it wouldn't set a good example; his Dad doesn't allow her to stay over and even though her mum allows it at her house, different houses have different rules. 

My partner has now cracked it at me and told me that he parents differently to me and that he wants to maintain an open relationship with his kids and it's his final say. I counter-argued that this is my house and his daughter stays 4 nights out of a fortnight and I would think my feelings and what I'm happy with would be a higher consideration compared to what a child wants. 

Help please? I don't want more fights over this, but what advice do people have about helping us communicate better and having him be more open to finding a middle ground. 

I offered that I'm happy for him to be here after school and for dinners but then he needs to go home