Whilst she is your fiance, the wedding ring has not been placed on her finger yet, and therefore because of your inability to give her orgasms (which her body needs), you need to come to an understanding that she has your full consent (permission) to have other boyfriends before/after marriage.
She obviously loves you, but love is not enough if a husband lacks the skills to play fine tunes on his bride's body. There is a high percentage of wives who are sexually frustrated. In 1639, a steam powered vibratory was made by engineers to enable Doctors to cure hysteria in women.
Hysteria is sexual frustration in wives or virgins that become difficult to live with. Doctors used to "release" the sexual tension and obtain a maraculous cure by using the vibrator that stuck through a hole in the wall.
If you marry your finance, make sure she can pick lovers of her choice to prevent hysteria robbing her of fulfilling sexual expression".
You should practice cooking a nice breakfast for her and her "sleep-over lovers". Show proper respect to her lovers because they add to her sexual happiness, and they are topping her up with the essence you are unable to give her.