It’s probably from reading magazine articles or your copy of 50 shades
Yes, that’s where men are getting the current obsession with anal and BDSM - magazines and 50 Shades... 🙄
Give over. He’s watching porn. Depending on his age he’s probably seen a lot of these types of acts - they are pretty mainstream in porn, and I’ve seen some people who are very into porn describe some of the acts as “vanilla” even since watching a lot changes your perspective on these things.
Have you been together a long time? If not, it’s possible he’s been building up to it.
If you’ve been together years, has anything changed lately that would lead him to watch more porn? (Eg less sex, pregnancy / new parents, breastfeeding etc)
Obviously if you’re up for it too, then great. But it should go without saying that just because he wants to do it doesn’t mean you have to do it. Some women love those things, others would hate them - either response is fine.
What is not fine is just doing new (and time especially more niche / extreme) things in bed without having a discussion about them first. I know that after having been in some sexually abusive relationships, I really struggle to say no to something as it’s happening - I’m more likely to freeze, or to just go along with it to prevent a problem (not that my DH would ever want me to do that, but it’s a hangover from my previous experiences).
If you’re the kind of woman who can say “no, don’t do that” as it’s happening then great but I think for most women it’s actually much more difficult to say no at the time than many men realise.
If you can have a discussion beforehand about your boundaries and preferences, it’s much easier to say that you’re not willing to try x but you’ll happily try y and z as long as he checks in with you / stops if you seem uncomfortable etc, or whatever works for you personally.