A man who is superior to yourself in several ways (either perceived or otherwise), who is currently dating a woman you are interested in. When he is better looking/smarter/wealthier/more educated/more adventurous than you are, he represents an idealized version of manhood or a picture of maleness that other men are expected to reach but realistically have little chance of doing. Hence, he is an example of Man-Jesus.
Yeah, I'd ask her out...but you've never met her boyfriend. He's Man-Jesus, I don't measure up.