I just wanted to pull on a particular thread from that messy ball of yarn on the front page.
Cocksucker is an insult because the act of sucking cock is understood to be submissive and demeaning. The cultural imagery supporting this interpretation is pervasive, from the 'money shot' in porn to blow job metaphors. Underlying that are the power-through-height dynamics buried so deeply in our evolutionary make-up that it's unlikely they will ever be eradicated, and which manifest themselves in everything from sexual relations to career prospects to architecture. The insult is usually directed at men because sucking cock is a woman's job and the worst insult that one can direct towards a man in most cultures is to suggest that he is in some way woman-like.
The use of cocksucker as an insult isn't just homophobic, it's patriarchal and misogynist as well.