Don't you think whoever goes out with XX would have fun everyday?
I think whoever dates XX would really enjoy themselves every day.
この場合、次のような言い方ができますよ。
ーDon't you think whoever goes out with XX would have fun everyday?
「XXと付き合う人は毎日が楽しいと思わない？」
to go out with ... で「…と付き合う」
ーI think whoever dates XX would really enjoy themselves every day.
「XXとデートする人は毎日とても楽しいと思う」
to date で「デートする・付き合う」
ご参考まで！
カナダ 
