Hi Wineman, welcome to the board.
If you're germophobic I'm surprised you were able to continue having sex on the sheets another guy had been in 6 hours earlier! But good for you. However, as others have said, it's OK to ask for a sheet change before coming over to have sex with her. However, she's not obliged to change sheets right after the other guy leaves and before you come, if it's just a short while. Maybe she's tired or busy and doesn't feel like it. In which case, renegotiate. Have sex elsewhere. Or wait another day. Etc. Whatever.
Personally, my partner and I don't have a sheet change requirement, since we know certain sex or kink acts don't even happen on the bed.
Of course, we try to be careful about where actual sex fluids go.
You started this thread by saying you were upset to know some first date guy was texting her hearts and "I miss you's." First of all, that's pretty forward behavior after one date. Hm. Secondly, how do you know he texted her this lovey dovey stuff if you don't look at her texts? Did she tell you? There was no need for your gf to tell you about this intimate (perhaps overly) romantic message! That's called being a sloppy hinge. Plus, while she probably found it exciting to be so desired by another after one date, you felt upset, insecure, to hear it. Therefore, you two need to decide what is TMI about her dates. (Or your dates, if you go on some.) Also, remember that the privacy of the other partner is also at stake. She should ask her other dates how much information about dates they'd prefer to have shared. Just because MayDecember reads his wife's texts and vice versa, doesn't mean many or most polyamorous people do. In fact, it's quite uncommon and considered a breach of privacy.
Many polys don't want any information about their partner's dates with others, other than, "We went to X restaurant. We went to his place afterwards. I had fun." Then if you're curious, say, if the relationship is new, you could ask, "Did you have sex? If so, did you use condoms?"
That is basically all you really need to know. Anything extra may be fun to hear about, or may cause envy or jealousy. You two decide. Sometimes if you're having a bad day, details of the others' fun might cheer you up, or might make you envious, you know?
If you're ever envious of your gf's other dates, ask for more dates yourself. Make sure to keep dating her, not just putting your relationship on auto pilot while she explores exciting NRE adventures with others.
Another thing to manage is time and money. Many of us polys have expectations about time management so all partners feel taken care of and at least mostly satisfied with how much time they get from their SO. And money, especially if you're a long term couple and finances are shared. How much money goes to dates with others?