I presume that we are talking about heterosexuals having platonic friendships with members of the opposite sex, yes?
There are some who overgeneralize and say that heterosexual men are incapable of being friends with a woman without wanting to get her in bed. Similarly, some say that heterosexual women are not able to develop friendships with men without developing romantic feelings for them. Personally, I am able to be friends with men without developing any lustful or romantic desiring for them.
People who think that others are so quick to fall in lust or love are likely projecting their own simplistic characters on others. Even in High School when I was far from emotionally stable, I did not fall for every guy I befriended.
As for what is “acceptable,” that is a subjective judgement which varies from one culture to another. If a person is concerned about what other people think about them, and they live in a socially conservative town, then they might find themselves the subject of gossip if they spend a lot of time with someone whom they only consider a “friend” – especially if that is time spent in private, when who-knows-what is going on. However, I believe that if you are an adult, what you do with your time and who you invite to your home is no one else’s business, unless they also occupy your home.
Times are changing and people are evolving to realize that humans and our relationships are complex and varied. If you find someone that you enjoy spending time with, then do so – regardless of what body parts they were born with, or what others think.