What has living with a priest got to do with it? I'm sure a few men live with other males with no intimate connections.
As a Christian, it is important not to be hostile to your neighbour so he is probably in favour of gay marriage so as not to oppress anyone.
But he and the priest would know it is against their religion to be gay themselves.
I don't know what kind of Christians they are but some denominations accept gay people and have churches that actively welcome gay people. Isn't it plausible that Cliff could be one of those?
Not that I'm saying he actually is gay, just religion doesn't mean he's not.