Your homosexual son or daughter is grown. They have a partner with whom they want to share their life with. These two decide they want to expand their little family (whether it be with a surrogate or adoption). Would you love your now grandchild any less? How would you treat your child?
This is the problem with so many well-meaning heterosexual liberals, and some conservatives too. They use heterosexual templates and copy them over to homosexuals. The heterosexual template results from a combination of male and female interests having to find common ground. If these well-meaning heterosexual liberals would just actually LOOK at how the homosexual male community CHOOSES to live their lives, they'd see that the couplehood and monogamy ideal is NOT the norm. Even when there is commitment to another person it, typical relationships have a mean "extra-marital" count of 6 sexual partners per year. Does that look like what the heterosexual monogamy template looks like? I don't know any young girl who is in love and wanting to get married who would be OK with the idea of each partner bringing 6 new people into the marital bed per year.
Homosexual marriage ≠ heterosexual marriage but with two people of the same sex. These are two different creatures.