Its not the sex act.....
And nature or God, whether you like it or not, designed males and females to be together. Because truth be told, we don't exist any other way.
Now now, classy. You can't go around sticking God or nature up people's butt just because your bright mind is clever at rationalizations. For millions of years nobody knew that sex and babies were related. Guys don't fall out of their tree when then see a nice ass, seeing oh my God let me make a baby, they are looking to pop their rocks for the orgasm, that big moment when they feel at one in enoumous pleasure. So by your reasoning if you have ever touched yourself you know exactly what it is to be gay and get off with a male hand. But I'm sure that with your great power of rationalization, you will find all that to be completely different. But I bet you, you first fell for a boy experimenting with what that thingi hanging there did besides excreting pee.
Nope, the fact of our design tells us nothing because the first thing you would have done as a boy, if you were double jointed is stick that thing in your mouth, and your Book be damned.
We are driven to have sex, not to have babies and never knew until recent history what the drive was for. We were driven long before we invented God or discovered nature. And there are selective advantages to gay people in a small social group. Humans do not depend on having large numbers of children to survive, but on those that are born surviving via social structure and group interdependence. Gay people mean brother and sister have extra help with the nephews and nieces of gay people. You may have also noted that a rather significant number of gay people seem to be gifted and contribute disproportionately to society.