he might be trying to make you jealous by talking about other women. or he might be trying to persuade you that he is clearly desirable to the opposite sex so you better hold on to him. he might have been trying to make you laugh by cracking a joke. he might have been...

i really don't think it is helpful to make generalisations of the form:

'what is wrong with men'
or
'what is wrong with women'
or whatever

because... any answer you are likely to receive will be unfairly overgeneralised and hence false for the majority of men (or women) in the world.

if you want to know what is up with him...

why not ask him?

he might not have realised that he was talking about the other girls a lot. if you had have said (or say if it happens again) 'do you realise that you talk about past girlfriends a lot' then i guess you could see whether he was aware of it or not. he might not know precisely why he was doing that... but if you tell him how he is behaving so he is aware of it and you tell him how it makes you feel then he might be able to refrain from doing it quite so much in future.