why do very unattractive men who I don't know keep trying to friend me on Fb?

[Avril]

[blackgrumpycat]

Because you don't have your profile set to friends only. Do that and the strangers won't try to friend you.

[Mido]

you may have sexy or attractive photo

[Info_Junkie]

I am with Mido. Men are visually attracted. So if you are pretty and additionally have provocative photos, then many will try to befriend you, especially unattractive males.

[George Bush]

Maybe you are hot?

Also set your privacy settings so that you don't get unattractive people

[...]

Maybe they like your profile picture.
try making all of your Facebook pictures and statuses to "friends only" including your profile pictures.

And change your Facebook settings to friends or friends of friends for requests.