Is it considered sex?

Depends who you ask, if a girl got a husband and some other guy inserted just the tip of his penis into her vagina, I think the husband would consider this as sex.

Yep. even if it's just the tip.

Sex is a lot more than just putting a certain percentage of a penis into a vagina. If you are having for example oral sex, then that is actually sex. Or if you are just putting the tip of a penis into a vagina, that is sex too.

It's the teenagers who think that only going all the way in for at least X times and getting an orgasm counts as sex.

Nah.

It wasnt against her will! OMG..Im just asking cus my bf before me said that he ALMOST had sex with a girl but that only the tip went in cus he was scared since he was at a public place, but that he considers me his first since we had actual complete sex, as in way more than just the tip. I told him that the way i see it i wasnt his first though..idk i'll just let him believe whatever he wants lol (;

Lets just hope that it wasn't against her will.

I'm not sure what the point of doing that would be, but I think it counts as sex. Not very satisfying sex, but sex indeed.

If my bf put the tip of his penis in my vagina and rolled over and went to sleep. I would not say that we had sex last night. No.

Well if it went in?

More like attempted sex. If there was no tweeeeet and chiirrppp and Oh MY FUCKING GOD!! then I don't think it was sex.

Maybe a quickie...?:)

Seriously, I would say that it depends on the context of the situation, for example what happened before this short genital contact. Let´s say that there was a lot of oral sex, manual stimulation of genitals etc., then I would be inclined to label this situation "sex" as a whole. Or if the described incident was a result of premature ejaculation, it may lead to pregnancy, so again I would probably call it sex.