There was a time when you and your wife would "double date" with her sister and her sister's husband, but for the past few months, you haven't been part of their double dating.
She knows how sexy she is, and how much it excites you to see her in a bikini -- and she has always found it amusing to excite you and then deny you while she goes off to have fun with other men.
How many nights started promising but ended in humiliation and disappointment? By this point in your marriage, it seems to be common place. Your wife is past the point of frustration and needs to be satisfied. You tried your best, but maybe it's time to step aside and let a more capable man take your place. Doesn't your wife deserve to be satisfied for once?