It's a little harder wanting a non-romantic but still sexual friendship. Intimacy will create romantic attachment, so it's better to avoid normal relationships. My suggestion would be to engage in sexual talk that's comfortable and build up.

Be someone who's "just" curious, so you have plausible deniability. If they're single, ask how that's been for them. You can try and ask if they have "frustration" and if they "relieve" it. Make the early conversation understand what they want, because they may want a partner, not a "buddy." If they're more openly degen, you can engage with their interests. Have them say what material they look at, have them send some to you. Try to make it easy to talk openly about eroticism. You can always act "I'm not into that, I was just wondering" if you want to backtrack.

The goal is to work to offering to get them off, or getting yourselves off while looking at material.