Having fear is simple human nature. Yes, extreme fear such as a phobia is often irrational. However, there is an extreme fear for almost everything including an irrational fear of Christians. Rightfully so, I would fear someone who yells at me that I'm a sinner and I'm going to hell. Life is already hard enough as it is and I don't need priests or pastors saying I'm going to a place of eternal damnation if I don't turn to God and tithe my income to the church.
I have never been assaulted physically or verbally in the name of gay sex, or for refusing to accept gay sex. I have, however, been assaulted physically or verbally by so called Christians in the name of their lord and savior, and for publicly saying I'm an atheist. I'd say Christophobia is not irrational.
Yes, Christophobia is a plausible fear to exist, like homophobia, because there is a great deal of anti-Christian sentiment that is out there. Some people fear Christians for no other reason other than that they believe in God. Basing a fear on someone just for what they believe is a concerning phobia that exists in modern culture.
"Christophobia" (which itself sounds like a term made up by bigots who were called out on using their religion to justify their bigotry) is an annoyance, at most. Homophobia contributes to the public perception that homosexuals are less than human, that it's okay to discriminate against them. So no, "Christophobia" is not a plausible fear when compared to homophobia at all.
The definition of Christophobia is anti-Christian sentiment is an opposition to Christians, the Christian religion, or the practice of Christianity. The definition for homophobia is an extreme and irrational aversion to homosexuality and homosexual people. To me, definition wise, I do not believe the two terms are comparable since Christophobia doesn't illicit the fear of Christians or Jesus Christ. I find that somewhat odd, but I believe in my opinion it makes the question false.