I don't think I'd be able to see it as cheating unless the person is actually getting off. If RPing sex is basically wank fodder for someone, then yeah, that's iffy, but that's also not what everyone RPs sex scenes for.

And what if the OP sees all of their partner's free time as couples-time? In that case I would agree that the partner taking me-time instead might be harmful for the relationship, but also that the relationship might be fundamentally unhealthy and not worth staying in.