Seamus I have no doubts that Ronin's answer was better :)
There were some reasons - one was traditional alliance and common interests (for example German colonies - without serious forces there.
Another was mentioned by me earlier. I'm just bad guy so I imagined what would happen if Portugal joined Central States. Far from any allies, without support. I don't know portuguese army level but I don't expect it was bigger than british. I'm not even mentioning fleet - uncomparable. Biggest portugese cities are next to sea - british fleet would destroy them.
Portugal choice was easy - supporting Ententa gave some benefits, joining Central States - only problems.