Two same-sex guinea pigs mounting each other means they are homosexual (and/or being sexual to each other) FALSE
Mounting, rumblestrutting, and any like behavior is normal when two or more guinea pigs are trying to determine dominance. It is not of a sexual nature. See
http://cavyspirit.com/sociallife.htm#introductions for information about introducing guinea pigs to one another.
It should be noted that male guinea pigs (neutered or not) will still mount a female guinea pig to try to mate. If the male guinea pig has just been neutered, wait three to four weeks before introducing him to a female or else you risk the possibility of mating due to surviving sperm in his system; anytime after that should be safe for introductions. If you have an intact male AND female (i.e., neither are spayed or neutered), separate immediately and if possible, neuter the male. See
http://www.cavyspirit.com/breeding.htm