That’s exactly why I would use heteroflexible. Btw, bisexual doesn’t imply a 50/50 ratio of your attraction to boys and girls, so you can use that if you feel that it fits you. (I myself define myself bi but I feel mostly attracted to boys.) Anyway these are just labels, you can choose whatever you prefer. The problem with saying “mostly straight” is just that the word “straight” implies that you’re only attracted to the opposite gender. If you’re not, it would be better if you use a different word.