It's crazy and irresponsible to spread a bug about if it can be avoided - which it can with this set-up. Sure ... in a "together" family, you'd have to take the risk that a bug's going to spread, but where there is an opportunity to keep germs away, because the children have another home, then surely to god it's sensible to do so. As far as the ex going out is concerned .... the weekends could be swapped about, so the OP's husband has the children, say, 2 weekends in a row once they're better.
It's nothing to do with dad taking the rough with the smooth etc .... it's about being considerate. There have been similar threads on here before about thoughtless, ill relatives descending on their families. This is the same thing.
Quite obviously, if a child becomes ill during a stay, then of course they must be looked after, but to insist on sending over an ill child regardless of the consequences is selfish. Additionally, like it or not, in step situations, where children have 2 homes, it's inevitable usually that the home where they spend the majority of their time is regarded as the home, and the place they feel most comfortable. And where do most people want to be when they're ill ? .... at home, in familiar surroundings - it's actually quite mean IMO to deny an ill child that. On top of that, if contact involves a car journey to and from dads (can be hours for some step kids)
I also think it's bloody selfish to insist your children leave you when they're poorly ... who wants a long car trip if they're feeling/being sick ?
My son is a stepchild and when he was ill I never insisted his dad have him regardless. It wasn't about making my ex "do his duty", or about how it affected any plans I'd made .... it was about doing what was genuinely best for my son, and that was keeping him in his own bed, with all his own things (inevitably most step kids will have less "stuff" at their 2nd home) and, about being a decent human being and not spreading D&V, chicken pox, flu or whatever around unnecessarily.
I'd be furious if my children (or myself for that matter) caught something relatively serious and/or unpleasant from an ill child - for example, a schoolfriend - whose parent had knowingly sent them round to mine. This situation is no different and objecting to it does NOT mean the OP is uncaring.