I would think our relationship should have continued more than two months, especially since he already met my parents.
この場合、次のような言い方ができますよ。
ーI would think our relationship should have continued more than two months, especially since he already met my parents.
「彼は私の両親に挨拶をしたんだから、私たちの関係は2ヶ月以上続くべきだった」
結婚の挨拶ではないのなら、単に to meet one's parents を使って言うと自然で良いでしょう。
ご参考まで!