We were caught in a compromising position in the stationary cupboard. Is this a sackable offense?

Whilst you are entitled to a private life at work, this does have limits, and an employer has a right to expect some standards of behaviour whilst you're on company time and on the premises. An employer could well view this as gross misconduct, and a reason to dismiss you immediately.

If you are summoned to a disciplinary meeting about this, make sure to discuss your options with your union rep if you have one. They are entitled to accompany you, and may be able to help plead your case.