Does it really matter if he's single?
Some women prefer to date men who are non-monogamous or involved in alternative relationship types, such as polyamory or open marriage. That’s perfectly acceptable.
However, if you meet a man you know is in a monogamous relationship and he’s pursuing something other than friendship with you, carefully consider your level of involvement. This is where it’s really important to know who you are and what you stand for. Be clear on what your beliefs are about sex and monogamy and relationships and especially about honoring the terms of relationships.
If you proceed with a sexual or romantic relationship with someone you know to be in a monogamous relationship with someone else, know the risks.