Is it forbidden to think about sex and discuss marriage and the wedding night with female colleagues at work?

A person is not held accountable for mere self-talk (حديث النفس), based on the Prophet's (peace and blessings be upon him) saying: "Indeed, Allah has forgiven my Ummah for whatever thoughts cross their minds, as long as they do not act upon them or speak about them." However, dwelling on these thoughts (الاسترسال مع الخواطر) can be a gateway to temptation and corruption, because they are seeds sown by Satan and the self in the heart, leading to intentions and resolves, and then to actions. As for discussing marriage matters and the wedding night with female colleagues without necessity, this is one of Satan's inroads that should be guarded against.