I must say that i cannot see your problem........having sex with someone different is not uncommon with many men. Going over the same old routine and receiving the same old response is boring.....just damn boring. It may be exciting originally, but if something "different" is not inserted into the equation, then one looks for alternatives and they are out there in abundance.
Not all men require satisfactory and fulfilling sex, but most need that in their lives.....different...fulfilling.....interesting.....exciting.
Fat women ?.........yes.......perhaps......
No, you and you perception of sex, is perfectly normal.