春 here is a classical euphemism for sexual activity — see also
春宫 (chūngōng, 'spring-palace,' meaning erotic art) and
春药 (chūnyào, aphrodisiac).
卖春 is a euphemistic, somewhat literary term;
卖淫 (màiyín) is more clinical,
援交 (yuánjiāo, 'compensated dating,' borrowed from Japanese
援助交際) is the modern slang for the milder end.
