Do you live together?
TBH, it's impossible to comment more without knowing more.
But the issue of women having platonic relationships with men (and vice versa) is a divisive one. Some people refuse to accept that they can happen. And some people get very unhappy about their partners seeing other men/women for this reason.
When you say 'secret dinners' what do you really mean? Was she deliberately concealing the fact that she was having dinner with this man, or were they occasions that you simply didn't know about?
I have a fair few male friends that I might have lunch with (occasionally dinner). Because I live with my DH, any evening out would be fairly obvious, and I would always tell him/he would always know, because it would be written in the family calendar! But if it was a lunch, he wouldn't necessarily know and, if he doesn't know the man in question, (i.e. it could be someone I work with/used to work with), then I wouldn't necessarily tell him either. But I wouldn't be trying to keep it a secret.
I can get that your partner is at a different stage of her life compared to you, in regard to parental responsibilities, but the most troubling thing about your OP is the lack of notice and consideration that she has shown you in telling you about this holiday.
I wouldn't, though, tell her not to come home. I would wait until she is back and then talk to her about your relationship and how she feels about you - and how this has made you feel. You could both be making wrong assumptions about each other and the relationship.