Yes, obviously the definition to a word is decided upon by the people, but it can't be manipulated by the people. Once a word has an officially accepted definition, you can't just go mutating that definition into whatever the hell you want. You can apply that definition however you want, but you can't alter it for your own purposes.
Just as many words, there are multiple meanings. For instance:
vir·gin ( P ) Pronunciation Key (vûrjn)
n.
A person who has not experienced sexual intercourse.
In my mind, sexual intercourse is a lot different than a sexual act, as you put it. You chose the loss of purity definition, I chose this one. Same word, different meanings, or contexts/interpretations for the meaning.