My 2000 Civic LX is bone stock (D16Y7). During cooler mornings (below 75 degrees F) the car cranks and cranks and will make attempts to start, finally kicking over after 10-20 seconds of cranking. Opening throttle impedes starting. Suspecting poor fuel delivery, I replaced both coolant temp sensors. No change. No running problems otherwise. Warm starts are fine, including after the car sits all day outside in temps above 75. Car has 116,000 miles on it, still accelerates fine and still gets 40mpg. This tells me that fuel delivery system is working fine (fuel filter/pump performing well). At a loss now. Thoughts?

You've done a good job analyzing the problem. You may need to replace the upstream oxygen sensor (Denso only):

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563.html#post223376

Have you checked for any "soft" codes?

Hmmm... I thought the oxygen sensors didn't come into play until after they warmed up. I was wondering if the MAP sensor might be playing tricks... I will try the O2 sensor. How does it play into the cold start equation?

Upstream oxygen sensors are the principal feedback sensor used by ECMs to calculate air/fuel ratios for fuel control. Intake Air Temperature (IAT) Sensors and MAF/MAP Sensors also provide the ECM with fuel control information.

Your question is a good one. I imagine the heated oxygen sensor must still be good to execute the "tabled" open loop algorithm. Your question implies that you don't need the oxygen sensor, while the engine is cold. My understanding is that reference and signal voltages to the oxygen sensor are just controlled by the ECM, until the engine warms up (2 minutes) but that the oxygen sensor circuity must still work to affect injector duration. I'm not knowledgeable enough on this detail to say one way or another definitively.

I'm with you, however, that I would first suspect and replace the ECT Sensor.