Different Gender, Opposite Sex
noun, の adjective
Someone who differs in gender from you is of a different gender. This is often used to mean the opposite sex in the sense of men as viewed by women and vice versa.
いせい
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
シェアハウスについて初めて知った時、知らない異性同士が暮らすなんてありえないと思いました。
When I first learned about share houses, I thought it would be impossible for strangers of different genders to live together.
今日は異性といつもより長く話しをしました。
Today, I talked to a person of the opposite sex for longer than usual.
美果は、異性の筋肉を意識しすぎてるんだって。
Mika, you are too conscious of boys' muscles.