I understand that people act against their best interest but in this scenario someone is influencing the politicians to act against their best interest (trains). In your example, the gay people are voting against gay marriage but someone has convinced them that gay marriage would be bad¹. If everyone loves trains, why would anyone try and convince the politicians to act against the trains?

¹ I don’t love this example and because this is evil autism I’m not going to follow social norms and will reply even though I know it’s mostly irrelevant 😈

There are 2SLGBTQ+ folks who are against gay marriage but not because they agree with the right wing. Some people reject marriage all together. Some people think the nuclear family is a mistake, and harmful. Some people think the benefits afforded to married couples (in some areas) should be available to all. Some people feel that when gay marriage is made permissible it enforces the expectation that people in non-heteronormative and/or non 1-on-1 monogamous partnerships behave in the heteronormative way, and that failure to do so means their partnerships are lesser. It might not be that they think gay people should not be married, but that they disagree with marriage and their vote against is a protest of the concept.

Personally I do think the nuclear family is a tool of capitalism and imperialism and for the longest time was against marriage. I did eventually decide on it because even in a place with low social expectations for marriage and a lot of rights afforded to common law partners, being married provided automatic rights in the case of medical emergencies and death.