Man or woman, we don't usually greet each other by kissing over here. I was at a Christmas party in Amsterdam last year with my dad's friends, having to kiss ladies on the cheek to greet them was very awkward. The worst part was that there were people there from all over the world, so different people expected you to give them one, two or sometimes even three kisses, and I never knew how many I was meant to do! :-D

Men normally don't kiss eachother in the US; that might be the source of some of your problems.

In Spain people give two kisses on the cheeks when they meet someone for the first time, ususally in an informal context, and between women or between woman and man. Men usually shake hands. Also it's common giving two kisses or hugs with familiy, friends...

But in more formal context, e.g. jobs, people of both genders shake hands. And perhaps in some occasions, give kisses. It depends on the level of interaction.

It's the natural form of greeting or meeting someone. In fact, in informal contexts it would be a bit 'cold' and less natural greeting only saying 'hello'. It's part of our culture. I didn't know that in Italy for giving kisses first is in the right cheek and then in the left. As you say, in Spain is in the opposite side and it can be confused haha.

In Greece people do not greet all people by a hug and a kiss on both cheeks.  This is reserved for greeting family members, relatives, and our best friends.  So in reality, very few people greet others like that.  Gender however, is irrelevant in these cases.  People are comfortable with that because they have known each other for so many years.  Greetings for everybody else other than the groups mentioned above is done with the shaking of hands regardless of gender.

In rare cases I have seen women in offices in Greece, to hug and kiss on both cheeks but again it is not common, and it greatly depends on the personality of the people and the level of familiarity they have with each other.  Guys on the other hand are more reserved at work. Women are more open to each other, and generaly more natural in their interactions.  In my opinion guys are more "forced" to behave in a certain way in the office.  Great respect is shown to elders and people of old age (especially to older people outside ones family) where in this case the greeting would be done by kissing ones hand.  It is not as common any more as it used to be in the past, but it is still practiced so it should be mentioned.

In Morocco, for greetings, man to man or women to women we greet by shaking hands or, most common, by kissing cheeks sometimes two kisses sometimes four it depends on how much time we  haven't seen each other XD

Man to women shake hands or just nodding. 

For old people we kiss their hands to show respect. 

Also after shaking hands it's common that people bring their hand to their heart to show respect and friendliness :D 

Please point out if there are mistakes in this paragraph. Thank you ^_^