You paraphrase Lumen Gentium 25 and (surprise, surprise) in a way that happens to suit your interpretation.
Go back to the exact text : "... definitivo actu proclamat." 

I happen to think that HV is a much misunderstood text. But that is in part because in too many countries it was presented as "Pope says, so no need to even think about it. Just obey. Cos it's infallible, 'cos the Pope said it." 
If it had been presented as deeply-thought out pastoral advice, then instead of the bald "this means no contraception" people might have explored more its pastoral core about the nature of love, and the relationship between sex and love.

In other words, I don't think wanting to say HV is infallible is helpful. Quite the opposite.