Don't be dumb. You could easily marry a divorced woman who had a cheating husband or one that was born with HIV. Or you could easily get HIV without sexual intercourse such as getting a blood test or even hair cut at an unhygienic establishment. My whole point is testing before marriage and informing your spouse before marrying again is a must!
You're a prime example as to why Somalis need sex ED. I'm embarrassed for you. To simply put it, we should normalise checkups before marriages.
Those reasons you've provided are nothing but anomalies and rare inconsistencies. STDs, and other sexual diseases are usually transmitted via Zinaa, or any other unlawful contact with the opposite sex. I'm not saying that they don't occur, but they clearly don't happen as much as sexual diseases and HIVs being transmitted via rampant Zinaa.
My problem with your statement was the conflation of Zinaa, what is considered unlawful in Islam, and Polygamy, what Allah (SWT) made Halaal. If they're 'cheating' on their wives, it's Zinaa, something Allah (SWT) made unlawful, and the punishment is the stoning of their bodies until death. It has nothing to do with Polygamy done in a Halaal way. Don't conflate the two.
Zina is Zina, and Polygamy (taking up to 4 wives in Islam), are completely different.
But we're on the same page regarding the consultation of one's family, spouse, etc. when seeking multiple spouses.