Let me offer you my welcome, as well, Krypto.
I agree with everything that has been written above, but I want to give another point of view and expand on something you, yourself, alluded to -- Pandora's Box.
I initially approached PM the same way you are -- with a desire to keep my male sexual function intact -- because I'm in a relationship where my partner wants to have vaginal sexual intercourse from time to time. We are in our 60's, post menopausal, so frequency has gone way down. At your tender age, being able to have normal man-woman intercourse is probably a much more important consideration.
Aside from suction gadgets that require hours of time to achieve results, I believe the PM is the best way to grow small breasts slowly and still maintain your male sexual function. Here's how:
Ramp up to a dosage of PM where your nipples start to tingle, ache or itch. Stay at that dosage, or even try coming down a bit if the feelings can be maintained. Along with the PM, take a daily dose of Butea Superba and L-Arginine. They will compensate for the negative effects of PM on your sex drive. You may not feel the urge to have sex as much, but you should be able to achieve an erection with a little foreplay and have intercourse with your lover. If the erection is a bit weaker than you're used to, and your doctor says it's okay, take a generic Viagra (sildenafil citrate) an hour before sex. I use Suhagra-100 that I buy on-line for about a buck a pill these days. If you have to increase your PM to get breast growth and you find it increasingly difficult to achieve an erection, take a PM break for one week out of the month. The key is to straddle the hormonal fence. You won't be quite as virile as you are now, but you will likely be able to get some breast growth. It will take many months to achieve anything that will fill up a bra, though, so you have to be very patient.
BTW, if you intend to have children someday, it would not be a bad idea to freeze some sperm in case you become infertile along the way. That's not likely with the above approach, but it is Pandora's Box after all. Once you start taking PM, it's not just your body that changes, but also your brain.
Again welcome, we'd love to have you join us on our trek through gender land. It should turn out to be an exciting journey for you.
Clara