Hey Dev,
Need not worry yourself with the intercourse part. I did not get graphic but there is a difference, I just said it can go inside, I did not say you could pump away like a mad man LOL and also getting it inside takes some extra effort but can be done, just please make sure to use a condom...hey get creative!!! ( Warning!!: DO NOT try and place an object inside the condom with the penis to give it a stiffer feel, it will puncture the condom and can puncture your partners internal parts!!)
As for the beach I assure you this is 100% safe with a waist band sewed to it. You may want to wear low cut briefs or something underneath your trunks if you feel better about it...the only issue you can have is looking well endowd after getting yoru trunks wet and coming out of the water...other than that there is no real issue. Some guys would rather wear low cuts under the trunks because it just feels better. I highly suggest trying this out in a pool, or whatever before going with family and friends...other than your partner. Just to find your personal comfort zone.
Peace,
Taylor