"乱搞男女关系的人" can be translated into English as "someone who engages in casual or promiscuous sexual relationships" or, in a more direct and perhaps slightly stronger tone, "a person who has indiscriminate sexual relationships" or "a promiscuous individual".
The choice of phrase depends on the context and the level of formality or severity you want to convey.
"Someone who engages in casual or promiscuous sexual relationships" is a more neutral and descriptive way to say it.
"A person who has indiscriminate sexual relationships" implies a lack of selectivity or discretion in choosing sexual partners.
"A promiscuous individual" is a more concise and possibly stronger term, often used to describe someone who has many sexual partners without much commitment or emotional involvement.