Not sure if this is a question of reasonable vs. unreasonable.

What about bringing him to social activities where there are many kids the same age? Perhaps you know families that have kids the same age? You cold invite them over occasionally.

Probably a healthy thing to do anyways.

This would encourage dialogue between not just you and your husband but other people in your community.

It might be worth while trying to find out why he is so keen to have them so far apart, if he has agreed to another one in theory.  Did he have issues with his own siblings as a child?  If you can find out the reason why he doesn't want two small children at home it might be easier to tackle the source of his opposition, rather than just trying to win him over with wine and sex (although that's not a bad idea either ). 

My husband and his brother were 6 years apart and although they get on fine now as adults, he says they never had any mutual friends or did things together as children or teenagers.  "Two only children who happen to be brothers" is how he describes his situation. 

I don't feel what your asking is unreasonable, actually I feel your husband is the one being unreasonable. Try keeping the lines of communication open without the subject leading to a agruement or he will dig his heels in and you will get no-where.

Your line of thought about children growing up together is the line you should stick to....two children growing up together, teaches them amount of things....sharing....socialising ( especially for entering school)...togetherness....friendship.

Maybe plan a couple of romantic nights with a nice meal, wine & even candles and when the  atomsphere is right approach the subject again on a lighter note.

I wish you well.

Regard Janice