I've felt tempted to ask for a zero penis policy... but I very quickly realized that it's not my place to do so, before actually bringing it up with my partner.
It's her sex life, and I knew right from the start that she's not 100% Lesbian (she probably wouldn't have chosen me as a partner if she were, anyway) - so the matter is simple: Her choice to make who she sleeps with, my responsibility to deal with any irrational feelings that choice causes me.
Did I tell her I probably can't ever be really happy to have a penis interact with someone I love, let alone feel compersion? Yes, of course I did. It's my duty to honestly talk about things that make me uncomfortable.
Was I relieved to hear from her that sex with guys is a lot less important to her than with gals, and "serious" relationships come almost exclusively with women for her (with an oddball like me thrown in)? Yes, of course I was. However, that feeling of relief told me to make sure she's not just saying this as compromise, though - I had to make sure that feeling of hers ("I don't really need sex with guys and could live without that forever") would be just the same if I never entered her life at all.
But it was it ever my place to tell her what she may or may not do? Hell no it wasn't. I don't have a veto over my partner's love or sex life. And if it causes me pain/discomfort, that's my job to deal with, not hers. It's not my job to limit her choices out of my own issues - that is not love, it's ownership and abuse. Love should enrich the partner's life, not ever detract from its choice of options.
So, can I understand why people suggest OPPs? Yup. Very much - though as an asexual, my reasons are probably different than a straight dude's.
But understanding isn't the same as justifying. OPPs rub me the wrong way for the same reason that monogamy does - interfering with the freedom of choice of another person out of some kind of "partner's entitlement". I won't stand for being treated that way, so I couldn't look myself in the eye anymore if I start treating a partner that way, either.
If it hurts, that's where the growth happens. And if it hurts too much, it's my choice and my responsibility to walk away, instead of starting to blame my pain on my partner's actions.