A. Where are homosexuals denied their right to due process?
Hard to answer that question directly and correctly since the question misses the point. Keep in mind that I'm not grandstanding for "gay rights"; I'm specifically supporting "same sex marriage rights." It's important to differentiate that since we all know that a homosexual man may marry the same people that a heterosexual man can and a heterosexual man is, in nearly 40 states, not permitted to marry the same people a homosexual man is not permitted to marry.
A man and a woman may consent to marry each other regardless of their sexualities, and I think everyone here agrees that they should be so permitted; therefore, sexuality isn't the issue.
It is important to know that the issue is that when there are two pairs of citizens each consenting to a social, personal, public union they call a marriage, each couple has natural rights that include but are not limited to property rights, personalty rights, child custody rights, etc. When citizens have rights, those rights are to be protected by their government.
When the government recognizes and protects the marriage rights of one couple but not those of another solely because of the sexes of the parties involved, that is a clear and obvious violation of due process rights. It is discrimination not of one's sexuality, but of one's sex.
B. What amendment to the Constitution guarantees the right to marriage?
The basic human right to consent to marriage is a Liberty
. We are so endowed by our Creator. Liberty is above all constitutions, rendering the fact that I don't know of an amendment to our Constitution that enumerates the right to marriage irrelevant.