The idea of Catholic priests and nuns being single comes primarily from some of St Paul’s comments, notably in 1 Corinthians 7
www.blueletterbible.org/Bible.cfm?b=1Crc=7v=1t=KJV#top, “It is good for a man not to touch a woman… nevertheless, to avoid fornication, let every man take a wife… but I would that all men would remain as I.”
The idea that Jesus can’t be married comes from the idea that sex is by definition sin. (drifter will say sex is fun!)
We see the same thing with Adam Eve stories, where it’s always been trendy to assert that “forbidden fruit” was a euphemism for sex, and therefore that sex is Catholicism’s “original sin.”
In Catholicism, Sex equals adultery equals fornication–and therefore if you’re not celibate, there’s no sin in the latter two.
And if they accept that, neither Jesus nor nuns can be married, because Jesus was “perfect,” and perfect doesn’t include any sex.