Isn't that just an excuse to decline the proposal?
You thought of that reason after she/he made the proposal, so that you can refuse it. Right?
ご質問ありがとうございます。
① "Isn't that just an excuse to decline the proposal?"=「それはプロポーズを断るための言い訳に過ぎないのでは?」
②は少し長くなってしまいました。
"You thought of that reason after she/he made the proposal,"=「あなたは彼女・彼がプロポーズをした後にその理由を考えた」
"so that you can refuse it. Right?"=「それをあなたが断れるように。でしょ?」
ご参考に。