I don't know how old you are or how tall you are but I understand that people don't believe you when you say that your member reaches your knee. I do. I am 5'4.5" tall, measured by a medical attendant. The distance from the base of my penis to my knee is only 12 inches. I am a little short on the ass end but I cope, we aren't all model material and definitely not made with the same physical proportions.
I work at different locations in town and consequentely have to use different facilities, some not as modern as others. I have come across toilets, depending on the make of the bowl, where I have dipped the end in the water. A creepy feeling because of cleanliness reasons but nevertheless a reality. I have simply made a habit of holding myself and no more problems, I don't even drag the lid with me as I stand up either.
Underwear is a problem as well. I have found the problems to be hit and miss and you will have to experiment to find what you require. I tried many kinds, tight , loose, boxer, and have simply settled for briefs by boulevard club. The leg band doesn't let me fall out and because I don't like tight underwear they work for me because they loosen up after they are on, adapting but holding, holding, not squeezing.
As for bicycles, I used to use the bike a lot but not anymore. I have read and have come to reason out for myself that when riding a bike the seat is supporting my weight and my weight is resting on all the valuable muscles, vessels and other tubes that nurture that particular part of the body as well as others. I walk a lot, it keeps me fit, I get where I'm going and my doctor supports me in that.
I used to be self-conscious of my crotch or showing my bulge but now I have found comfortable underwear and clothing and when my crotch settles as I walk and do other things, well that is just the way it is. I used to try to adjust and position myself but that can be more attention getting, even stimulating.
These problems are ones that can be fixed and understood. Enjoy and let yourself be aware of and appreciate the gift you have. I hope I have given you some insights and I sincerely hope you don't find your endowment a burden, but admirable.
Take care.