Archer, there are as many perspectives as there are mongers, but I wanted to share my perspective to this oft discussed issue because we have some similarities. I once had a beautiful girl at home who also had an indiscretion relatively early in our relationship. In our case, we never fought over it because I actually understood why it happened and it had nothing to do with our relationship or her desire for another man. I had hobbied somewhat before we met but stopped when we started dating and ultimately restarted after ~18 months of dating. We are now happily married, love each other completely, and I am also very comfortable in my hobbying.
The reason the mental image of swingers as two naked flabby 60 year olds is that it takes many people 40 years to shed all of the religious grief regarding monogamy that's been beaten into us for the first 18 years of our lives. There is no magic answer to your feelings of guilt as the issue is not as simple as choosing to accept or deny any social view of P4P hobbying. I'm guessing you're in your 20s or early 30s so I'll state that living without guilt comes with age/maturity where you become comfortable with yourself as a (hopefully good) person. Sex was, is and always will be one of the most universally pleasurable things an adult can do so finding an enjoyable sex life is a good thing for your mental well-being. Hopefully it still involves your SO (unlike many guys on this board) either with or without extra outside action.
The beauty of the P4P world is a no string attached relationship where, hopefully but not necessarily, you like and respect your hourly companion, but your love and focus is completely at home (as is hers for that matter). If your secret ever does come out - and regardless of what guilt you feel, it should never come from you - then do you really think it will help her feel any better that you went to other girls and only got (1) naked body rubs, (2) naked body rubs and HE, (3) only got a CBJ, (4) only got BBBJ but CFS, (5) only got BBBJ, BBFS, but not anal, etc? I think most women will feel betrayed at #1 so the question of hobbying or not really comes down to you, not her. If your relationship is meant to be, she will be in a position to understand your lifestyle choice just as, it seems, that you came to grips with her indiscretion.
IMHO, your real problem is straying with female acquaintances as these situations are fraught with real emotional/relationship betrayal unlike P4P sex. I know that my wife would be infinitely more devastated if I had a real affair versus if I went to Germany to tour all of the FKKs and had sex with 40 different 21 year old women in one week.