Your characters can be in an environment that looks down on public affection. Big business, military, clergy, even academy (professor/student, not so much professor/professor) can all be very restrictive in permitting any kind of relationship to show at work, and even outside of work. But great relationship do start even in those frosty environments, and it is not uncommon for two people having mutual sympathy to get caught in a moment.

Or your characters can be gay and don't know it yet :)

Your characters can be in an environment that looks down on public affection. Big business, military, clergy, even academy (professor/student, not so much professor/professor) can all be very restrictive in permitting any kind of relationship to show at work, and even outside of work. But great relationships do start even in those frosty environments, and it is not uncommon for two people having mutual sympathy to get caught in a moment.

Or your characters can be gay and don't know it yet :)