Many men have learned to oblige this pretense by pretending to offer a prospect of romance even when that's not how they feel. The result is too many relationships that begin with lies and end in tears. It actually takes far more communication and work from both partners in order to keep it going. Assuming you and your partner can in fact agree on your goals and be honest about what you're doing, there are other issues. Again this is mainly a problem of romance.
Boundaries are less an issue in sexual friendships and almost a non-issue in recreational sex.
Polyamorists take advantage of an important loophole in this constraint. While it's deadly to try to run two romances at once, and probably not possible to be in two limerence episodes at once, it's perfectly possible to have a stable post-limerence romance established with a primary partner and to have a limerence episode with a secondary going at the same time. The challenge is to bring that limerence episode to a soft landing, modulating it into a long-term sexual friendship rather than letting it crash in a painful breakup. With awareness, discipline, and a supportive primary partner, this is possible. I have indicated the importance of honesty and making sure you and your partner keep your goals matched.