[If homosexual relations (for example) are against God's law, then would you believe that not only gay marriage would be wrong, but any kind of gay sexual activity? If so, what penalties do you think would be appropriate for breaking this law?]

Well the Bible calls for death but he who is without sin and all. I only think they should be fined or jailed.

[Other than being unConstitutional and impossible to enforce, not to mention making criminals of normal people...]

Its not normal. We put people with mental disturbancies in asylums don't we? As homosexuality is a mental disturbance they should be put in asylums for mental help and spiritual guidance.

[How would we go about legally handling the issue of straight couples who engage in "homosexual sex acts" and homosexuals who engage in acts shared between heterosexuals?]

Assuming they don't get caught there's nothing the government can do. If they get caught thought then they should at least be fined and probably have mandatory therapy.