Your common sense flies in the face of
evidence.
Even so, you are defeating your own argument now. If humans alone have intellect and conscience, then how is homosexuality "unnatural"?*
Aren't you really merely arguing that only the opinion of your intellect and conscience (or those who agree with it) count in establishing morality?
Why are you opposed to allowing some people to use their own intellect and conscience?
Are you the only human with "common sense"?
*ETA: In other words, if this is the reason for not using "nature" to consider infanticide to be moral, why should we ever turn to "nature" to answer questions of morality? If you're only talking about human nature, then the argument that homosexuality is contrary to human nature is demonstrably factually false. (Homosexuality actually exists, both in non-human and human nature.)