This is one problem with every discussion on the subject. In the broadest sense, it is always fornication which is the sin where any sexual act is forbidden. Neither Scripture nor Christian tradition contain the concept that "gayness" or homosexuality (as a character trait) is a sin. At most, it is a characteristic which might make it more of a burden for a person to remain chaste.
The reason being reproduction is unattainable through this method of intimacy.
Christians in some denominations may disagree, but I see absolutely no basis for that. If that were the case, sterile or post-menopausal married people would be sinning by sleeping together, while fertile adulterous couples would be less guilty than those using contraception. The likelihood of reproduction does not effect the sinfulness of a given sex act. The central, almost the only guideline for Christians is that people can have sex only with their own spouses.