couples who get into a relationship soon after meeting
知り合ってから付き合うまでの期間が短いカップルは同じ外見レベルを相手に選ぶ傾向がある。
There's a tendency for couples who get into a relationship soon after meeting to be people at about the same level in terms of looks.
looks = 容貌
「level of attractiveness」も言えます。
「soon after meeting」 = 知り合ってからすぐに