Except that’s not fully true in the Legal sense, though it may be in the Catholic sense. Children born into a marriage that is later annulled are still legally considered legitimate, and legitimate children require having been born into a marriage. There are also other parts where the legally binding nature of a marriage will still connect the two, depending on what they did while married.
In a marriage annulled because of impotence, where are the children going to come from?
As to the rest of what you wrote: the marriage is considered valid until proven otherwise. Suppose two people, both virgins, marry, and then discover that one is incapable of consummating the marital act. They may decide to see if treatment will help, etc., and then in the meantime, they buy a house together, pay taxes as a married couple, etc.
Because the marriage is considered valid until ruled otherwise, what they do before the annulment is considered valid as well. If the treatment never works, and the couple decide to separate and seek a civil annulment, yes, what they did while married will need to be dealt with.
If the annulment occurs for reasons other than impotence, and there are children involved, the same holds true; however, this was not always the case. Legally, there was a time, and it is still true in other countries, when children of a marriage later declared null would then be considered illegitimate, as indeed they technically were.
Either a marriage which is later annulled has to be considered as valid while it is happening, in which case the actions taken, including having children, must be dealt with as having legitimately occurred during marriage, or it has to be considered invalid, which causes a basic level of uncertainly wrt marriage which should not exist. Societies used to be more inclined to the latter view, which caused problems; now many societies are inclined to the former view, which seems better overall.
However, the fact that society is trying to be more fair should certainly not be considered supportive of a major change in the definition of marriage such as is proposed under SS"M".
The Church teaches that children born of a marriage later canonically annulled are legitimate.