If, after all that has been said to indicate that the reference to the portion of the body that sits upon the seat of a motorcycle is in no way, shape, or form a reference to anything sexual, salacious, or naughty, and you still have to use a _________ to substitute for the term, then perhaps that indicates being too sensitive.
There's not much difference between an adolescent boy snickering when a donkey is referred to as an "ass" and a grown women getting the vapors over the use of the word crotch in reference to where a motorcyle rider's body interfaces with the motorcycle. Neither is a mature, appropriate response.
Do you swoon when the four upright portions of a table that support the table top are called "legs"?
Oh, George, there's just something gross about that word to a lady. There are words I hate that are used to describe the female anatomy, too. By all means, have your boyish fun. If we want to use an underline because we actually were taught decency and modesty as ladies, then let us be. 
Kim, there are degrees and levels of "decency and modesty". I am only suggesting that in the year of our Lord 2010, the quaint and excessive standards of "decency and modesty" common when HRH Victoria was Queen of England are no longer appropriate. The days of going into a swoon because of a word like "crotch" when referring to a, well, crotch, are long behind us. I'm 59 years old. I was raised by parents who were above average in terms of personal ""decency and modesty", and I know that neither of them, nor either of my grandmothers, both of whom were born around the turn of the last century would get the vapors at hearing the word "crotch" used. I can recall when I was a grade school kid back in the 1950's when I was being taken to the local store to buy some back to-school trousers and my grandmother told the salesman to be sure the pants "fit in the crotch".
But, I'll off you and anyone else who is so sensitive that she cannot bear the word "crotch" when used in a non-sexual setting this simple proposal. Tell us what euphemism you would prefer to be used in place of that objectionable word.
Suggested alternatives include loins, groin, lap, pelvic girdle, and pubic area.
And for the record, I am not attempting to decree how anyone should feel about any particular word. If some people are excessively sensitive about certain words even though such excessive sensitivities are, well, excessive, I am not saying that they are wrong to be overly and excessively sensitive. If anyone wishes to be overly and excessively sensitive about certain words, then that is what they are. It would be no more appropriate for me to attempt to force someone to get over their extremely excessive over-sensitivity to certain words than it would be for the overly sensitive to attempt to compel me to accept and agree with their perceptions. Excessive over-reaction to particular words is neither more nor less a sign of superior character or qualification than a relaxed, tolerant attitude is.