下記のような説明で分かるでしょうか？
relationship material = A status bestowed from one individual upon another exemplifying satisfaction anticipated from maintaining a relationship with that individual. This evaluation is highly subjective and typically reflects a great deal of personal preference. However, this term is usually used in situations where the relationship is not exclusively sexual.
ようするに（個人的フィーリングで）付き合いたくなるいい奴だということなので、直訳的日本語はどうでしょう。ウマが合う相手、とでも言いましょうか。
