It is up to you.
Is this a HARD LIMIT? Like polyshipping is just NOT your cup of tea and never will be? No amount of time will ever change the fact that for you, you like your romances to come in a monoship shape?
Is this a SOFT LIMIT? One that could change in time because the problem is not polyshipping in general but HOW she is doing it right now? You have objections to some of her polyshipping practices?
Since sex is the trigger... is it her moving too fast into sexual relationships before you can know/trust the person with your beloved? Unsafe sex practices that put you at risk? Something else? Is the sex share poking you not in your "Physical health bucket" but in your "mental health" bucket -- arousing jealousy things
like fears of abandonment or other kinds of jealous
like how you fit into her life NOW?
If you know deep down it is a hard limit... let her go. As hard as it is, choosing an end point and short term break up suffering is better than no-end-in-sight continual suffering.
If it is a soft limit and you really want this for yourself too, perhaps schedule an honest talk about the HOW she approaches polyshipping right now and what could be improved so you can feel physically safe, emotionally safe, mentally safe, and spiritually safe. So you can work toward being comfortable and happy in polyship.
Hang in there!