Welcome to the forum. Thanks for your question.
I strongly urge you to not let this be a big deal for you. Trichomonas has always had the somewhat peculiar aspect of a "bimodal" age distribution: if you draw a graph that shows the frequency of trich compared with women's age, there is a peak at age 20-30, then the frequency drops, then there is a second peak at age 45-60. Most or all infections in the first group have been recently acquired, almost entirely by sex with infected men. Most of the second group -- i.e. cases like yours -- have no apparent STD risk. As you learned in your own research, the best bet is that in many or most of those women, the infection has been present but dormant for many years.
Transmission by hot tubs, towels, shared undergarments, etc is all nonsense. Such stories arose because people didn't understand the issues I have just described, so people tended to look for explanations other than sexual transmission. It's an urban myth.
For these reasons, even though trich is generally sexually acquired and transmitted, your new diagnosis doesn't necessarily mean a new infection. Ask your gynecologist: I'm confident s/he will have run into this problem farily frequently over the years. And even if this is a new infection, it doesn't necessarily mean your partner has had other sex partners recently. He may have been carrying trich for many years, and only recently transmitted it to you. You're a much better judge than I can be about whether he is being truthful about it. But if all other evidence is that he is, then you should trust him.
Testing for trich in men is very unreliable. The standard tests usually used in men miss most infections. His negative result doesn't man much. He probably has it and needs treatment to prevent you being reinfected once again.
So as I said above, please don't be upset about all this. You aren't alone, and I doubt your partner has been unfaithful. The most important thing at this point -- and probably the only important thing -- is that you both be treated. And by the way, although single dose treatment with metronidazole (Flagyl) or tinidazole (Tindamax) is the norm, single dose treatment is only about 90% reliable; treatment for 5 days is more reliable. And in men, tinidazole appears to be more effective than metronidazole. You and your partner should discuss these aspects with your doctor.
I hope this has helped. Best wishes-- HHH, MD