Oh gosh why are some people on here so horrid? Is this how anyone would really respond to a friend with a four month old baby who was exhausted and asking for advice?
OP- Your feelings about weekends away will be entirely different depending on your baby’s temperament (looking after a sleepy little one who coos away happily and plays contentedly is entirely different to a teething colicky baby who frets and won’t sleep!), your baseline energy levels (how much sleep you’re getting, how much your DH normally does his share in terms of parenting, how much of a break you’re getting regularly), your relationship dynamic and the availability of your own social circle.
You’re a new mum (again) and given what you’ve said it seems your partner is not taking account of the fact he’s also a new father (again) with young children.
If it is a very close friend could he go to the smaller stag then one night of the big one?
Two weekends in a row if he was consistently providing 50% of the childcare, overnight duties, etc, You had family who could come stay over if you were feeling overwhelmed- Yes fine.
But 5 out of 7 weekends away with a four month old and two year old at home - and when it sounds like you’re tired, don’t have friends/family nearby and just need some help - absolutely not ok.
If we’re playing top childcare trumps, my XDH was in the forces and missed the first year of two under two. I wouldn’t recommend it. I still think OP is not being unreasonable. Stags are essentially men getting very drunk and I’m not sure why OP’s DH doing this two weeks in a row for the same friend should be considered more important than OP’s feelings and wellbeing.