You say you want to talk about the issue rationally but you immediately claim as fact things that are not true. You claim marriage is "an outgrowth of biology, anatomy, physiology, the obvious means by which Homo sapiens as a species is perpetuated." In other words, you believe marriage is about sex.
However, there is a long history of recorded same-sex unions (see Same-sex marriage: the personal and the political By Kathleen A. Lahey, Kevin Alderson) and heterosexual marriage has evolved over time (see also The Evolution of Marriage By Charles Jean Marie Letourneau).
Marriage is not about sex. Marriage is a union between 2 people who promise to love and care for one another above all others and it is legitimized by the state which, if a license is issued, will confer a legal status that comes with specific rights and benefits.
Gay people are legally able to have sex with other consenting adults. We are not asking for a right we already have.
When you go to a wedding, you are not there to witness 2 people who will be having sex from this day forward until death do they part. In fact, sex is not mentioned in traditional wedding vows at all. When you celebrate your grandparent's 50th wedding anniversary, you are not celebrating their 50 years of sexual activity. You are celebrating the anniversary of their union and the fact that they were able to sustain their commitment for so long.
Finally, your supposed request for dialog ends with a reference to bestiality -- thereby equating the desire of gay men and lesbians to create legal protections for their partner and children with people having sex with animals -- which is a form of abuse as animals are incapable of consenting to the sexual act nor are they able to enter into a contract -- which is essentially what marriage is.
It is insulting to gay men and lesbians to compare our struggle for equal marriage rights with bestiality.
If you truly want to talk reasonably and rationally about equal marriage rights for gay and lesbian couples, I suggest you re frame your thesis.