So far you can only chose to enter a romance route when it first becomes available.
Viewing post in Rise of the White Flower comments
I do, but it's hard to just give you a list because there are way too many variables involved in what scenes you do and don't.
Lena, which is the earliest romance route you can access, allows you to sleep around no problem, so you don't lose many scenes at all. Evelyn, Yda's, and Amelie's (to be added) romance are also open. Eleanor's, Khai's, and Marius (to be added) are closed relationships so you mostly just have access to their scenes. Other romance routes are still undefined at this moment.