I define sex much as Adriana suggested: sexual contact with another person in which I could potentially contract or give a disease, including when safer sex practices are implemented. My definition is certainly not limited to penis/vagina penetration. It includes oral and manual stimulation, some forms of tribadism, among several other activities.
Generally I do not tell people the exact number of sexual partners I have had. I am regularly tested for diseases/infections & practice *very* safer sex so the NUMBER of partners I have had is not particularly relevant in most situations.
I take a don't ask/don't tell approach with my actual number of sexual partners. Quite frankly, I don't want to know the names of every person my partner has been with or the number of people. There's nothing I can do to change it, and it's not something I want to dwell on. If my partner's number is higher than mine I tend to view his/her as promiscuous, and if his/her number is lower than mine I feel like a slut. I know this reaction is not logical, but I also know from experience this is the reaction I will have - which has been to the detriment of some of my relationships.
The exception to all this is when I decide to be fluid bonded to a partner. Then there is a serious discussion outside of the bedroom about previous sexual encounters, disease testing, and what the limits of the fluid bond will be. At that time I will generally tell someone my number is less than - and more than - and ask they do the same.
**Please note that you can get a sexually transmitted disease or infection from performing and/or receiving oral sex. A woman can also be infected with some diseases if her partner uses his/her hand on her genitals or in her vagina and the partner's hand/fingers have open sores, cuts or any other lesions. (This includes finger penetration of the anus for both men and women.) This could actually include irritated hang nails.
I feel it is important this thread have this information as the general population does not seem to know this very serious information.**