I do my best to avoid religion, but here goes:
Do you feel that sex outside of wedlock (fornication) and cheating on your wife (adultery) are sins?
According to both the Old Testament and New Testament, both sex outside of wedlock and cheating on one's spouse are adultery. And, both are sins. However, both the Old Testament and New Testament continue arguments and reasons as to why these are viewed as sins; and, not surprisingly, the accounts diverge.
The Old Testament cashes out this sin in terms of honor. By sleeping with a woman and not marrying her, one dishonors that woman's family; by cheating on one's wife, one dishonors one's wife (and, also, that woman's family). Moreover, the sexual act itself is viewed as inherently unclean, both within and without the marriage covenant, in the Old Testament. Sex itself is viewed as something that forces a later cleansing process for the Israelite.
The New Testament, on the other hand, views sex as something that holds large sway over the individual, clouds there minds, and brings there thoughts away from God. Again, it does this both within and without the marriage covenant. However, marriage is recommended because marital sex results in less cloudiness (basically, when one is monogamous, they no longer chase after every sexual urge). Thus, sex outside of marriage keeps one from focusing on God. Cheating on one's wife, however, is the breaking of a vow not only to one's wife but to God (as the marriage vow asserts a fidelity to one's spouse, and that assertion is made to God).
This covers the basic views of sex that come out of the Bible. This also gives us reasons to condemn homosexuality, from a Biblical point of view.
Feces are viewed as more unclean than vaginal fluids. Thus, the Old Testament view of male homosexuality (they interestingly don't provide a view of female homosexuality) as wrong can reasonably be defended as wrong because it is so very unclean.
On the New Testament view, the case can be made that this urge is less satiable than the heterosexual urge, in that the heterosexual urge likely results in children and a family, and the raising of children and having a family dampens sexual urges (it certainly does during certain periods of pregnancy and post-pregnancy).
But, the New Testament largely remains silent on homosexuality. In fact, the New Testament, in speaking of homosexuality, is strictly speaking to the practice of master-boy pederasty, which was not uncommon among Greek gentiles.
That said, we can address both of these views, from the Old and the New Testament, with a contemporary understanding. We know that both honor and uncleanliness played a large role in the lives of individuals living in 500 BC Babylon and Judea. This is a historical fact. We know that uncleanliness is the prompt behind certain Old Testament laws that we now very readily reject (prohibitions on certain types of foods, clothing, behavior during menstruation, etc.) We very readily reject these statutes because developments in hygiene render these statutes meaningless.
As for honor, this is an idea that has largely fallen away with the inception and development of Christianity. Christ did not give a **** about honor. He dined with the poor, the tax collectors, etc. He directly defied the notion of honor in parables regarding banquets and places at the table.
Homosexual acts, with today's advances in medicine and hygiene, are no longer a threat to the physical health of a community. We are not eating out of common bowls on tables with our hands; we bathe regularly, rendering innocuous physical contact innocuous. STDs are certainly transmittable, but between full-fledged homosexuals, such STDs only affect the homosexual community; further, outside of the homosexual community, STDs in the heterosexual community affect much larger numbers and do so in a manner much less conspicuously. So, the unclean argument really fails in contemporary societies equipped with advanced medical technology and personal hygiene methods.
Now, since the honor and unclean accounts are basically moot today, we are left with the "homosexual urges must keep individuals farther separated from God" reasoning. The burden of proof placed upon this stance is enormous, and the more common practices of adoption and birth control, the reasons seem unlikely to warrant any objections to homosexuality that cannot be leveled against heterosexual marriages (the Catholic Church, with its prohibitions on some birth control methods, abstaining). We can readily assume homosexual couples that adopt children will place the energy into raising such children as heterosexual couples do; and, thus, we ought to infer that energy will lend itself to a reduction in libido and sexual urges; those very urges which keep one from focusing on God. Further, we can readily assume that married heterosexual partners can indefinitely delay the having and raising of children, thus never decreasing those same urges that keep them from focusing on God.