When is the right time to tell my girlfriend I have a lower chance of getting her pregnant?

We've been together for 6 months and she mentioned early on about wanting to get married one day and have kids. She was very verbal about her goals, values and timeframe. I have issues with my sperm quality. I tested myself after having two past long-term relationship and never getting any of them pregnant. They're both already with kids from another man.

There are very few healthy ones and so the chances of getting a woman pregnant is greatly reduced. Imagine your Internet is running so slow that it takes forever to download a page. That's how it is with me. I can still get a girl pregnant but it can take much longer than usual. The quickest way would have to be through extraction; basically via a medical procedure.

When is the right time to tell my girlfriend this sensitive topic? Should I've mentioned this earlier or not is it still too early?

When is the right time to tell my girlfriend I have a lower chance of getting her pregnant?
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