It's not a black and white issue. The thing with evaluating morality is that it's circumstantial. Most will agree that murder is wrong. But murdering someone in legitimate self-defence is morally acceptable, and even expected. There are many factors that are to be considered when determining morality.
Does she know he's gay? If no, then perhaps it is not. But even then: is his life, well being, and/or freedom at stake if he tells her and/or does not marry? If this is the case, then an argument can easily be made that she ought not be told, and he should marry her. It's easy for most westerners to judge, forgetting that there are still many places where being gay is an automatic prison or death sentence and not marrying brings great suspicion. This one reason many gay men ended up in the clergy not so very long ago (and even today, really). Seminary was an acceptable reason to not marry, without arousing suspicion.
If she does know he's gay, and there's an agreement between them, there is no problem. Such marriages were not uncommon when my father (67 now) was in school, in the 60s–especially if she were lesbian. I think there's even a term for this, but I can't find it.
Morality is dynamic, as well. It's based on social norms, which change over time. In the early 19th century, it was immoral for a woman to let her ears show. less than 100 years later, she could show her ears, but not her ankles. Within 30 years, we were in the Flapper era. Grandmothers were utterly aghast at what their grandchildren were wearing (or, in their minds, NOT wearing).
I know that seems off topic, but it's to illustrate that morality shifts--often very quickly, within one generation.
