I think this depends on what angle you want to use the pregnacy, if you want to use it as a fetish and do stuff with them when they are pregnet than is a bad ideea to exculd one afterall everyone has a pref char and this will be better to let the player to select witch girl to get pregnet...But if you want to use pregnacy just at the end of the strory then its ok afterall we can imagine that at the end somehow that girl still got pregnet.I have one character specifically in mind that I have toyed with the idea of them not being able to get pregnant. I won't say who, but it's not obvious. I don't want to spoil anything, and of course it's still being thought of.
I think this decision will undoubtedly make some angry or sad, but a story needs ups and downs.