While 75% of men always reach orgasm during sex, only 29% of women report the same. In addition, most women are unable to climax through vaginal intercourse, instead needing clitoral stimulation. -Source: National Health and Social Life Survey
The best promise is NO promise. No need to keep secrets and no need for promises. These only open bad doors in your relationship. Say what's on your mind and relieve the weight. :]
PS: Here's a little joke..
When I was born God gave me 2 choices. I could either 1) have a great memory OR 2) be great in bed. Oh crap! Now I forgot what I was gonna tell ya! ;)