Because cultural change precedes political change, and the Left is currently winning on both fronts, we cannot afford to surrender to it on the culture wars, which include the homosexualist agenda of gay marriage and adoption.
It is true that the anti-Islam coalition is broader than the one against cultural Marxism, and includes homosexualists in it, but the latter are unreliable allies. They are against Islam not because they understand or care about the threat Islam represents to the West, but simply because they are only concerned about their own immediate interest, which is the normalization of homosexuality.
For that reason, because they do not think of the far-reaching consequences of the Islamization of the West, homosexualists are anti-Christianity as well as anti-Islam.
By demanding marriage and adoption for gays, homosexual activists know that they can reach and influence the new generations into their way of thinking.
The number one leader of the UK LGBT movement Peter Tatchell (who incidentally advocates lowering the age of consent, repeatedly defended man-boy sex, supported relaxation of laws against pornography "arguing that porn can have some social benefits, and he has criticised what he calls the body-shame phobia against nudism, suggesting that nudity may be natural and healthy for society") publicly says that, without repression of homosexuality and what he calls "heterosexual proselytising", everyone or nearly everyone in society would be bisexual:
Q: What's the difference between heterosexual and homosexual sex? A: A few inches of flesh...That's where they want all of us to get to.
If homosexual desire is this widespread in a homophobic society, imagine how much more common it would be in a gay-positive culture. With the cultural taboos removed, nearly everyone would savour its delights.
I wonder how many other "delights" Tatchell has in store for us, once the new situation of the "cultural taboos removed" will give him free rein to imagine and publicly profess support for more of them.
I suspect that many people who support or at least do not oppose same-sex marriage are not aware of the long-term goals of the LGBT movement, and of how its goals are incompatible with those of the majority of the people.