You don't seem to be worn out by domestic life so I assumed you were single!
You didn't seem to be too immersed in domestic life so I thought you were single!
「所帯じみていないのでずっと独身だと思っていました。」= You don't seem to be worn out by domestic life so I assumed you were single! / You didn't seem to be too immersed in domestic life so I thought you were single!
「所帯じみている」という表現は英語で「domesticated」や「immersed in domestic life」、「become housewifely」になると思います。
他のボキャブラリー
seem = ~のように見える
worn out = 疲れている
assume = 推定する,憶測する
single = 独身
immersed = 浸された