What do you consider to be "losing your virginity"?

This is just because I'm curious. I have had this debate with several friends and even in a human sex class I had in college. Answers range from "it's only sex if penis enters vagina" to "even heavy petting is sex".

So what is your opinion?

My personal opinion (for myself) is that sex is sex. But, just because somebody is comfortable engaging in one type of sex does not mean that they are comfortable in other types of sex. And I also think that there has to be another person involved, but that's just me personally. I do not think that I could take my own virginity by fingering myself or by using a dildo or a vibe on myself. If somebody disagrees with this, are either of us wrong? Not necessarily since the idea of virginity is such a social construct, just we have different opinions, that's all.