Present arguments that show what's wrong with cohabitation between a man(boyfriend) and a woman(girlfriend)

Con didn't provide any evidence to support his claim.

This is mostly a forfeit debate from both sides. Con, however, had the entire burden of proof in this debate. The only thing that con argues is Unmarried cohabitation relationships are built on sex and Sexual foundation is unhealthy. Con cites no sources nor does he give any reason for this. Pro points this out in the next round and shows that con needed to provide evidence for that. Because con failed to meet his burden, pro wins.

Equal forfeits - pro doesn’t offer any specific arguments other than a rebuttal for cons argument on all cohabiting relationships being sexual in nature. As such Neither side fulfilled their BoP.

This is 85% what an FF on the part of Con. Pro's later (but not R1) only forfeited Round is negligible as there was nothing more to reply to than in the previous Round.
Con loses because Pro counterargues that sex itself causes, not just results from, attachment. Con is left without a remaining point against cohabitation that I can see. Con inherently conceded that if there is attachment, said relationship is 'healthy' meaning that Pro's defensive argumentation style didn't need offence as Con had admitted that if attachment is there, the relationship is acceptable and that cohabitation is fine.