Have you ever heard of a homosexual having a fake boyfriend/fake girlfriend?
In the minds of many, being a single adult is “weird.” And, it’s one thing to be single, but quite another to be perpetually single. It’s not just, “why aren’t you in a relationship?”, it’s “why are you never in a relationship?” So, I think for many it raises questions as to your mental and emotional stability, your interpersonal skills, etc.
This is true of Catholics, even. Believe it or not! There was a Catholic Answers blog post not too long ago that chided single people for not moving towards marriage, with the core idea being that people have standards that may be unrealistically high for potential spouses. It spoke of the idea that there is “no single vocation.” (Which makes me wonder what the heck SSA people are supposed to be doing, and whether the Church even has a role for them or not. But that is another discussion.) So a single person is an oddball, according to many.
Add to this that the candidate appears to be somewhat conservative, and thus probably has to appeal to the “family values” crowd, which is just like any other group - there are many among them that are just plain ignorant and will question him because he doesn’t have a family of his own.
Add to this the pain of living with this secret. When everyone is wondering aloud, to your face, “why don’t you find a spouse?” I’m sure it is painful to keep covering that up, it must cause a lot of distress.
Add to this the need to “control the message” as another poster spoke of.