When a family oriented sim has a spouse, it usually raises the threshold at which all romantic interactions from other sims are accepted. To get them to try for a baby, not only does their long term relationship have to be very close to max, but they have to be in an exceptionally good mood.
A nice way to do it is to find out what the sim's day off is, and plan a day of it, starting after he gets off work on the day before. Invite him over and get him to stay the night, then spend the rest of the night doing something fun. Don't forget to cook something if he gets hungry (it's easier to cook before he gets there, and just take it out of the fridge as needed). Feed him the next morning when he wakes up, and allow him to use the bathroom and to shower. Once he gets all of his needs taken care of, he'll be more apt to accept.
Or...you could just buy the Master of Seduction LTR. With that, you can successfully try for a baby with complete strangers if you have access to a bed.