Contrary to the likes of luke evans, zachary quinto and matt bomer (who are all known to be totally gay) there are rumours of Pedro having affairs with women and in my opinion, if he was indeed only gay there would be 0 or just one or two stories but there is even a small list of women with whom he suposely hooked up with (lena headey being one of them. People that worked bts on game of thrones said their affair was known). There is also his old personal blog where he implied to have had a one night stand with a woman, so, yes, he's most probably bi.
But I do share the opinion of some that he's more into men