I actually understand him. Co-worker, 10 years younger (aged less than 20-22) is hitting on me = MAJOR PROBLEMS
Options:
1) Soft rejection - While talking mention you have a girlfriend; Mention you like men;
2) Ignorance - Despite seeing her affection, play the fool - reason being, eventually she will give up
3) Accept - Potential problems with her being a colleague, 10 years younger especially ages under 25 are risky.
But this is a wish fulfillment manga though i do not understand for whom. Shown as seinen but this author shows mixed signals for the target demography. Art seems for men/boys, content seems more for girls/women.
Author is a women so i kinda understand why there are mixed signals but i think this content would be more in tune with female demography.
While some men appreciate female attention generally speaking getting INTO relationship with someone much younger than you sounds like a pain especially girls before 25y. (If you are not an asshole this doubles the stress and problems)