I think it's very rarely the case that snarkers claim that fundies think all sex is bad. A huge theme on snark subs is talking about how obsessed with sex fundies seem to be after marriage, and snarking on things like Jim Bob and Michelle humping each other in front of an unmarried couple who are barely allowed to touch each other. Or all the talk about being joyfully available. Those are more Duggar specific examples, but those same themes come up all the time in FSU. Snarkers are very aware that fundies are pro-sex in marriage.
And that's part of the problem for a lot of people, especially women, who do find it difficult to make the transition from how sex is perceived when you're unmarried vs. married.
And this whole conversation is also ignoring LGBT people, who definitely don't get any positive messages about their sexuality from most Evangelical churches.