We largely agree on obesity it sounds like, it's a self-control issue.
To some extent, yes. It can be complicated, but people do have some responsibility for it. How much can be debated, but again, it's beyond the scope of this thread to have that debate.
However, it does relate to the thread because it's a comparable example. It's an example of Churches enabling (and even celebrating) things that go against some of the extraneous rules listed in the bible.
Matrimony is a sacrament instituted by Christ Himself, and moral law regarding marriage and sexuality is far more deeply rooted than you're suggesting. This is not about "extraneous rules listed in the bible". You can equivocate between bake sales and gay marriage as much as you want, but it doesn't make the one the same as the other.
If someone feels that homosexuality is a sin and that a church officiating a wedding for it is "embracing" or "celebrating" that sin.
Then the same would have to be said about a church advertising a bake sale, and willingly selling a cheesecake to a church member with a 52" waistline knowing that it could contribute to the sinning of a member.
Having a bake sale is not a sin. Selling food is not a sin, even if that person is obese. Heck, being obese is also not a sin. Sinful actions can lead to being obese, but being obese is not a sin; in the same way, extra-marital sex is sinful, but being pregnant is not.
As for "contributing to the sinning of a member", selling food to an obese person who
might be indulging in gluttony is no more a sin than providing alcohol to guests at a wedding is a sin because someone there
might drink too much. You might as well say that having any kind of social gathering at all is encouragement of sin because someone there
might get in a fight, or gossip, or whatever. That would be a ridiculous thing to suggest, and I think you know that.
All that said, gluttony is frequently a venial sin in Catholic reckoning. Same-sex marriage is not. They're not on the same level.
But by that standard, a gay wedding wouldn't really meet that criteria either.
It's not denying a core doctrine, and doesn't involve either participant rejecting the faith.
Speaking from a Catholic position: I can't find a definitive answer about whether or not it's heresy in a strict sense, but it does have to do with rejecting definitive Church teaching regarding one of its Sacraments. Even if it's not
technically heresy, it's right there.