I am not going to sleep with someone who already has a girlfriend.
彼女いる人とセックスしません。
I won't sleep with someone who has a girlfriend.
I am not going to sleep with someone who already has a girlfriend.
~とセックスしません
I won't sleep with~
ご参考になれば幸いです。
I'm not going to have sex with someone else's girlfriend.
It would be wrong for me to have sex with someone's girlfriend.
We are waiting until we are married.
If you mean not having sex with someone else's girlfriend then saying, "I'm not going to have sex with someone else's girlfriend", or "it would be wrong for me to have sex with someone's girlfriend" should settle it.
However, if you mean you and your girlfriend are not having sex you can say, "we are waiting until we are married" or "we've decided to wait", or "we will wait until we are married."
他の人のガールフレンドとセックスするつもりはないという意味なら、次のように言えます。
"I'm not going to have sex with someone else's girlfriend"（他の人のガールフレンドとセックスするつもりはありません）
"It would be wrong for me to have sex with someone's girlfriend"（他の人のガールフレンドとセックスするなんて間違っています）
----
ただ、もしあなたとあなたのガールフレンドがセックスをしないということなら、次のように言えます。
"We are waiting until we are married"（結婚するまで待ちます）
"We've decided to wait"（待つことに決めました）
"We will wait until we are married"（結婚するまで待ちます）
If you have a girlfriend, I cannot sleep with you.
I don't mess with cheaters.
To sleep with someone is an informal way of saying to have sex with them. Also if someone says that they don't mess with someone, it can be used to say that they won't sleep with them or they don 't want to interact with them at all.
To sleep with someone' は 'To have sex with someone'（～とセックスする）のインフォーマルな言い方です。
'I don't mess with someone' は「～とはセックスしない」「～とは関わりたくない」と伝えるときに使えます。
I don't sleep with people who already have a girlfriend
I don't have sex with people who are already taken
When talking about having sex with someone we usually just say to 'sleep with them'
If someone is already in a relationship or has a girlfriend/boyfriend then we also use the term 'taken'
人とセックスすることは、普通、単に 'sleep with someone'（～と寝る）といいます。
既に交際している人がいることは、'taken'（既に恋人がいる）という言葉でも表せます。
I don't sleep with anyone who is already spoken for
I avoid the situation of sleeping with anyone who is already in a relationship
I don't sleep with players
If you want to say that you don't have sex with people who already have a girlfriend, then any one of the above comments hits the nail on the head.
A PLAYER is someone who just aims at having sex with various partners without making any committment.
If someone is SPOKEN FOR or IN A RELATIONSHIP, it means they are already in a romantic partnership with someone.
彼女のいる人とはセックスをしないと言いたいなら、上記の文はどれも核心をついています。
'player' とは、責任を負うことなくたくさんの人とセックスしようとする人をいいます。
'spoken for' または 'in a relationship' は、既に交際している人がいることを表します。
"I don't sleep with people who already have a girlfriend"
"I don't have sex with people who are taken, they have to be single"
"It is not in my nature to sleep with someone who already has a girlfriend"
If you wanted to explain that you do not have sex with people who already have a girlfriend, you could say this very simply and clearly by stating any of the following: "I don't sleep with people who already have a girlfriend", "I don't have sex with people who are taken, they have to be single" or "It is not in my nature to sleep with someone who already has a girlfriend".
彼女のいる人とはセックスはしないと説明したいなら、次のように言えます。
"I don't sleep with people who already have a girlfriend"（彼女のいる人とはセックスしません）
"I don't have sex with people who are taken, they have to be single"（恋人のいる人とはセックスしません、フリーの人だけです）
"It is not in my nature to sleep with someone who already has a girlfriend"（性格的に彼女のいる人と寝ることはできません）
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