Love Square (2005)

Miki works in an office. She's dating Ryosuke and they're happy, except Miki is also in a relationship with her college professor Tanaka. Tanaka is trying to break up with Miki because of his wife, but Miki won't hear of it. Ryosuke has a one-night stand with Chihiro, who also has a boyfriend. The plot thickens further when both girls discover they're pregnant, and don't know who to name as father