The Watchtower definition of fornication (porneía) has changed over the years, and quite illogically homosexuality and bestiality were not grounds for divorce in the 1970's then 1983, a complete flip.
Watchtower 1983 12/15 p.27
"That fornication (which in the Bible sense includes adultery and gross sexual immorality such as homosexuality) can break up a marriage. Scripturally, the innocent partner has the right to divorce the guilty one and remarry without sin in God’s eyes."Watchtower 1/1/1972 p. 32
While both homosexuality and bestiality are disgusting perversions, in the case of neither one is the marriage tie broken. It is broken only by acts that make an individual “one flesh” with a person of the opposite sex other than his or her legal marriage mate.