People resist gay marriage because:

1. They are personally or emotionally uncomfortable (Logically Invalid)

2. Their religion defines it as wrong (Prescriptive but logically valid)

3. They disagree on biological & medical grounds (Logically valid, with complexities)

4. They have Ethical or Moral objections. (Logically valid, but extremely complex, more so than #2 or #3)

There are no other general reasons for rejecting ANYTHING related to our human sexuality including prostitution, beastialogy, nor pedophilia Think about it logically.

Now, you didn't ask WHY gay marriage SHOULD be rejected or accepted but to consider this corollary to your question: since #1 is an invalid argument only #2, #3, and #4 are valid reasons. There are complexities for all three. For #2, which faith system is really TRUE - they are typically mutually exclusive so, for example, both Islam and Christianity can't be true - they can both be wrong, but only one COULD be true, not both. #3 would include evolution, human reproduction, disease, health concerns, as well as physical birth defects. Psychology would also be involved, but arguments get complicated really quickly. For #4, the foundation of ones ethical and moral system make this exceedingly complicated. Philosophy, religion, law, common law, etc over the history of mankind really complicate this reason - but it is a legitimate reason.

Homosexuality is wrong according to an honest assessment and an authentic view of the following faith systems: Islam, Christianity, Judaism, Bahá'í, Jainism. There are probably others. However, if you are an atheist, then you won't have any such prohibitions to homosexuality or gay marriage.

As for argument #1, I agree; if a person authentically buys into a system of faith where homosexuality is defined as bad (Islam, Christianity, Judaism, Bahá'í, Jainism, others) then for them, it is bad; Regarding argument #2, yes, if a person has no external reasons to reject homosexuality (and thus, gay marriage) they problably would be persuaded to be more open and accepting if they were exposed to some sincere, kind, and loving gay people.

#3 is not a valid, logical, or reasonable argument. It is an "ad hominem" logical fallacy at best and will only create conflict, not solutions.