Matthew 1:19, Divorce, and Virginity In light of Joseph being called a righteous man in Matthew 1:19, does this mean that divorce because one of the two was not a virgin at the time of marriage is biblically valid?
I know of no doctrine held by any Christian tradition based on this passages that speaks to virginity at the time of marriage in relation to possible divorce. It is simply not the subject matter of the passage and drawing such a conclusion from it would be bad hermeneutics. You would need to find other teachings on previous relationships or extra-marital relationships in order to address that issue in a Biblical fashion.

If you look at other Biblical passages, you will note that basically the only valid grounds for divorce is the case of adultery. Even then it is not prescribed / required, only possible allowed.

If you look into the marriage traditions of the time, you will find that the issue of divorce was as relevant to Betrothal (engagement of sorts) as it was to marriage. A betrothal could not just be broken off for any reason, it was as good as a marriage vow as far as being binding on the parties, but the relationship was not to be consummated until after the wedding.

Mary getting pregnant during the betrothal was (barring the supernatural) obvious proof of adultery. Having committed herself to marry and be faithful to one man and entering a binding contract with him, Joseph would know he hadn't slept with her so she must have slept with another man.

With this understanding, we see that the only thing this passage could teach us about divorce is to reinforce the teaching that says adultery is possible grounds for divorce, but that it is not unlawful to NOT divorce even cases of adultery.