Rope has many uses and is bought by men ans women.
Just don't ask the clerk if it is strong enough to hold your weight...
Exactly....it's a common sense really. But there is a plethora of reasons to buy rope for : camping, hiking, construction projects, scouting (e.g.: to use for the Pioneering Merit Badge), heavy duty clothes line (throw a tarp over it to dry the tarp). Also to make a good 'ol fashion rope swing with a car tire.
Usually rope is rated to hold a maximum weight - maybe tell the clerk you want to use it for an informal tow for a small tractor. He or she should be able to advise—and if the clerk seems like some dumb kid (think of the someone like the "Buckley" character from the animated sitcom "King Of The Hill") typically the weight limit should be easy to find on the manufacturer's specs which they should have, and if not, then a simple Google search via one's phone should find it.
But I get what you mean about the feeling of being self conscious about it. If memory serves, I believe I read in Dereck Humphrey's book "Final Exit" (the updated version) that when the helium bag method started to become the more common choice among the elderly to ctb, there were a few accounts that some establishments were actually decling to sell helium to senior aged customers on the speculation they were going to use the helium to take their life.
Imagine the insult / embarrassment of being and older man or woman and you are buying the helium to inflate balloons for a party for your grandchild and the clerk at the store says "No, we think you might use the helium to kill yourself and we won't get our tank deposit back!"
Just FYI: the formula in the helium tanks underwent a change as to be ineffective for carrying out ctb.