If homosexuals are unsuitable to be priests, then how are they suitable to be husbands and fathers of children?
It depends if they were an outright, practicing homo or just someone with inclinations and effeminacy. These types of people, either way, are unsuitable for both priesthood and marriage, but...someone who has just "inclinations" is not necessarily prohibited from these sacraments. What I mean is that such inclinations/temptations aren't impediments, sacramentally speaking. Though we all know, in practice, these traits are a disaster. That's why you prepare in seminary for 6 years and why you date - to be on the lookout for timebombs.
I would imagine, if it is grounds, it would be based on hiding a material fact from the spouse which is critical to the primary purpose of marriage.
If he were a practicing homo before and after marriage, then probably grounds since he didn't have any intention to be faithful. But if he did want to stay faithful, but she just found out about his inclinations later, even if he had cheated on her, then I don't see how it could be annulled. A seminarian in the same situation would be a valid priest, it's just that in former days, were his inclinations to be found out, he'd be "retired" to either an ecclesiastical prison or a monastery on some island.
Much like a man who lied about being a baptised Catholic got married on that basis and then later admitted he was a fraud.
That's different, because it's a noted impediment to marriage.
Let's imagine a man was a convicted paedophile got out of jail and hid that from his future wife. Surely that would be ground for an annulment. He hoodwinked her. As I understand it you cannot hoodwink a person into marriage.
As bad as that is, immorality isn't an impediment to marriage. Prayer and due diligence would have these items surface before marriage, as God has promised.