There is a nuance to diplomatic trading that has me playing the game in a way I'm pretty sure the designers did not intend.

I tend to pursue peaceful victories, so my standing with other races is very important.  (Gotta keep them happy.)  To this end, Open Borders is a "must have" treaty with all races.  Since the game gives me a "timeout" on trading every time I make a successful diplomatic trade with a race, this causes me to decline every single trade proposed by other races.  The reason is that if I accept the trade I'll start the timeout which may overlap the expiration of my Open Borders treaty.  (And I have lost more than one game when this happens and my positive 80% rating goes to negative 80% in 4-5 turns because I lose the open borders treaty with an aggressive military race.)

I'm pretty sure I shouldn't be refusing to trade simply because a silly game mechanic forces me to keep my trading window open?

To be honest, I don't understand why treaties expire in the first place?  If I have open borders, exploration, reasearch, etc treaties, leave them active until one side of the other revokes them!  I find it tedious to have to constantly renew treaties with each race every few turns.

Nor do I understand why we have to have a timeout after successfully crafting a trade.