I would say that it's a "huge" part of his life and you've only seen the tip of the iceberg. You need to be with someone who is more vanilla.

I met a transsexual woman who used cross-dress back in the day when she was a he. Finally she wanted a sex change and it was too much for the wife and their 20s-something children. They ended up ostracizing her completely. Very sad for everyone involved.

The only way this would work in the long run is if you were geneuinely into BDSM or another such persuasion. You're not. You need to respect your limits and stop dating this person.

It'll hurt, but it's kinder to both of you in the long run.

If you don't believe, you two should see a sex therapist and really talk things (and the consequences! and implications!) through.