Why was the word "bull" taboo in some dialects of English? What did it mean?

The clue comes in the actual quote:

Word History: In many American regional dialects, the word bull, meaning "adult male bovine," was once highly taboo. When speaking in mixed company, people would substitute a variety of words and call the bull a booman, brute, gentleman cow, or surly. In the Northeast in particular, critter was a common word used to avoid saying bull, both by itself and in combinations like beef critter and cross critter.

It was taboo as the only point of a bull was to inseminate the cows. So obvious is this that there was a verb "to bull" (now obsolete) that meant just that:

OED

bull, v.1

†1. a. transitive. Said of a bull: to gender with (the cow). Obsolete.

1659 J. Howell To Knowingest Kind of Philologers in Proverbs sig. a4, in Lex. Tetraglotton (1660) He that bulls the Cow must keep the Calf.

1.b. intransitive. Of the cow: to take the bull, to desire the bull. Also to go a bulling. Obsolete.

?1523 J. Fitzherbert Bk. Husbandry f. xxviiiv The damme of the calfe shall bull agayne.

1601 P. Holland tr. Pliny Hist. World I. 224 Kine commonly..seeke the fellow, and goe a bulling againe.

This sexual implication of the word was simply "too much" for genteel ladies and euphemisms were introduced.