No, it's not an act. Maybe you enjoy it but I don't know why assume that just because you do that others feel the same way. I know men who would feel upset if they were groped by strangers so it's not a gender thing, it's about respecting the personal boundaries of others.
People have boundaries and they should be respected. When in doubt, don't infringe upon them or ask first.
For most people, it's a violation of their privacy. It's their body and they are entitled to have a say in what others do with it. It's not ok to simply touch a stranger in that manner. For people who have been raped before it can also be very triggering for them.
Even if someone enjoyed being groped by strangers, they may not be attracted to you and not want to have that sort of physical contact with you, they may not be attracted to individuals of your gender, they may have a partner and not be OK with having sexual contact with anyone else. The list goes on.
In short, everyone has boundaries so just because it's not a problem for you it doesn't mean that it's wrong or weird that for most everyone else it's a problem. As I said, if you don't know if the other person would like to be touched in that way, don't assume, ask them... that is what words are for. If it's not appropriate to ask then it's probably not appropriate to go ahead and touch them either.
Not sure what's so difficult about that concept.
It's like saying "I don't have a problem with strangers breaking into my house, why do people feel upset if someone goes into their house uninvited, are they just lying when they say that?". Makes no sense.
Don't be surprised if your entitlement to touch the body of someone else in a sexual way makes them entitled to punch you in the face