There's no need to choose the sex of the person, as you'll fall in love and your choice in the matter will be taken away. Whether they're male or female will depend only on what gender that particular person happens to be. That's unless you do decide that you're going to narrow it down, first of all. Personally, I wouldn't do that, as you might miss out on the right person.
I've been in a monogamous relationship for 23 years and never once strayed or even considered it. I can honestly say that I only find one person in this world attractive, in a romantic way, and I don't even think about any other people like that. I appreciate beauty and nice personalities, but I've never had the urge to get closer. The fact that my relationship is with a man is circumstantial and I would have been in a relationship like this with a woman, had the same person been female. The only difference would have been that I would still have wanted to become a mother, so we'd have had to come to some sort of arrangement on that.
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"We act as though comfort and luxury were the chief requirements of life, when all we need to make us really happy is something to be enthusiatic about." Charles Kingsley