In that scenario lying may be better for both in the short term, but lying about being in love with someone to trick them into sleeping with you seems pretty likely to upset them more in the long term. And there are more gentle ways to put it which could make honestly explaining that it's mostly a physical thing which would reduce the immediate negativity considerably, though the amount depends on the listener's disposition.

I agree that it's not necessarily unreasonable for a truth to be upsetting, but it is somewhat unreasonable to press someone for a truthful answer (especially something important), then be upset with them specifically for being honest, especially if they have indicated discomfort giving a direct answer and tried skirting around the subject (since this hints that it's something which may be an uncomfortable truth they may want to avoid), even if it's pretty common in many circles.