As-salamu ` alaykum,
The general principle is that we don't discuss our sinful actions with others, even prospective spouses. The Messenger of Allah (peace and blessings be upon him) said, "All my Community are well, except the revealers." When asked who the 'revealers' were, he replied: "Those who sinned and slept in the covering of Allah, but then tore it off and revealed their sins."
However, if people have exposed to any infectious diseases, then they owe it to their prospective spouses to get tested and to be honest about the results of those tests. Other than health considerations, however, it is best to not dwell on the past and to, instead, focus on a brighter future, insha`Allah.
I'm not suggesting that people lie to their prospective spouses if they ask, but sins that we have committed and sincerely repented for, meaning that we no longer are engaging in the sinful activity after asking for Allah’s forgiveness, are as if they never happened.
Hope this helps,