My metamour wants to be sexually involved with me, but not romantically

How do I accept that my fiancées new girlfriend might not want to be romantic with me and only sexual?

You're never obligated to "accept" something that makes you unhappy, if "accept" means just going along with another person's demands even if they're not healthy for you. If being sexual but not romantic with someone doesn't work for you, it's okay to not be sexual with this person. 

It's also okay to ask for clarification: what does this person mean by "sexual" and "romantic"? What are their expectations, needs, and desires? What expectations of yours are they willing to commit to meeting? What can they not do for you, or with you?

Remember that someone else's feelings don't reflect on your value or worthiness as a person - it doesn't mean you're someone not romantically interesting, just that this one individual doesn't want to be romantically involved with you.

Keeping in mind that people are not need-meeting machines, it's still very okay to actively pursue other romantic relationships in situations like this. Or, pick up a new hobby or make new friendship connections so that you have something to build you up.