It's difficult to answer because Sexual Magic(k) is a pretty big field of the occult. Like all energies it would have to be focused on a particular aim to be workable through a for example, a spell.

It doesn't mean that all acts of sex are tapping into a magical force - just that it can be used that way. So I reckon a fleeting thought probably won't necessarily manifest a material outcome. Be aware though that as the climax approaches then you might associate the experience with an unbecoming thought.... But then at that time you really shouldn't be having such thoughts, should you?!?