Why is my male guinea pig always trying to mate with the other male guinea pig?

Hello! my 2 male guinea pigs and they are being really wierd! i found 1 of them on top of the other yesterday! ........ not to be grose but his penis is coming out when he trys and i know its normal for a male to do that but its gone way to far! Theres goop coming out of his penis! and they have begun to fight alot!!!!!!! What can i do for it? PLZZZZZ HELP! and they are both boys they have been checked by a vet!!!!