Oh gosh, no, hun, your husband is the one being irrational...or really, and I'm assuming here that this is your first, he just doesn't get it yet or realise how involved parenting has to be. My daughter is nearly 2 and she's never stayed overnight with anyone yet except my husband and I. We've visiting my family in another month, which will be just after her 2nd birthday, and she might, might, as I'm still not 100% comfortable with it, stay with grandma for a night while we get a night out and stay over in a hotel. Our daughter never even stayed with another person during the evening so we could have dinner until she was 7 months old - and that was with my mum. There is no way in heck I'd ever leave her with someone I barely knew, even when she's like 8! At 5 months, they are still so vulnerable and need a lot of attention at night, which even if you assume it would be okay for her to stay overnight with someone else, probably isn't fair on these distant acquaintances as it's likely they might have to be up all night with her. I wouldn't expect that of anyone else personally. I think it's all fair to say it's nice to have a night out and you ought to do that if you have friends or family nearby who you know and trust with her. But I wouldn't feel comfortable leaving a baby that age with anyone, but especially not strangers.
In this case, I would offer him the chance to go on his own and enjoy a night out and stay over with his work friends if it's important that he goes and stays the whole night. No one could legitimately think you're being a jerk for being at home with your child in a situation like that. My husband went by himself to a close friend's wedding for the weekend when our daughter was about 4 months old. There was no way we could leave her as we don't have family nearby or anyone who could even watch her, not that I would have wanted to anyway, and it was a no kids wedding so we couldn't bring her. Or alternatively, you could ask someone you trust to watch her in the evening and just go for the dinner bit and take a taxi home. That way you can support him, but still be home to do the nighttime parenting. Yes, it's nice to have time away together as a couple, but I think there's a time for that, and I know at 5 months, we hadn't reached that point where it was possible. I think he probably just needs to be a bit more patient and understanding that time's have changed and things don't work the way they used to - though I assure you, you will be able to do those sorts of things again one day and enjoy them because you'll feel comfortable with who is taking care of your LO.