I'm split on the tantra thing. Sure it can be spiritual but I've run into way too many poly people that take it to the fanatic level. A kind of odd superiority that makes it almost cult like. Not everyone of course, and the same thing can be said for so many kinks and religions. However, personally, I"m not interested in it. So if I'm dating or considering dating someone who is, they need to be respectful of my not wanting to join them on that journey just as I'd respect them taking the journey. He wants to study tantra, he has a partner for that, great! If you don't want to then you don't need to. If all he is looking for is another tantra partner or to try and get you to 'convert' then the answer is a simple no.
Try and take the sex out of it and think of it as a partner with a hobby or religion you aren't interested in. "I'm glad you like it and find fulfillment in it. It's really not for me." I don't have to follow the religion of my OSO to appreciate that it works for him. If he felt I did, we wouldn't be dating.
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year