The opportunity cost to a tall person of being asked to reach something for a short person is generally quite small. This remains true even if there are many short people who require such assistance.
The cost to a woman of having sex with every man who asks, in terms of pregnancy risk, disease risk, and status loss from being perceived as promiscuous is rather more significant.
I can see why an objectivist would try to include them in the same reference class; a utilitarian, on the other hand, doesn't have to reach far to find reasons not to.
OrphanWilde also made an example about writing Web pages, whose opportunity cost is much higher than for reaching stuff but much lower than for having sex, so there might be more of a continuum than you realize.
P/S/A: There are single sentences which can create life-changing amounts of difference.