Let's actually go through some female champions and put your accusations to the test.
A fox transformed human by a dying mage. The transformation is incomplete and she is forced to steal the life essence from others to survive (she finds men most easy to charm.) She realizes that its wrong and seeks help from the greatest wizards in Runeterra. Not at all defined by her "relationships" but rather her moral struggle to balance her need to steal life essence and her desire to live.
One of the most badass martial artists in Valoran. Only mention of relationships in her lore is that she works with Shen and Kennen as one of the shadow warriors to enforce the balance of Valoran.
The eternal guardian of the Freljord. The only other person mentioned in her lore is Ashe, who Anivia allied with to protect the Freljord.
Dark child born in a magical place to two very powerful wizards, making her ungoldy powerful. Only people mentioned in her lore are her parents, who are only there to explain why Anne has such profound magical gifts.
Became the leader of one of the many tribes of the Freljord at the age of 15. Her story parallels that of King Arthur with her discovering the legendary bow of Avarosa and uniting the tribes peacefully under her rule. No mention of any relationships though her old lore did say she married Tryndamere to help unite the tribes.
Single handedly cleaned up the streets of Piltover and made it one of the safest cities in Valoran. Only person mentioned in her lore is her unidentified arch-nemesis who leaves ornate cards with a 'C' on them at every crime scene.
Short lore discussing how she became half woman, half snake. No mention of any relationships save that she's the youngest daughter of an influential family in Noxus.
A free thinker and scholar who was branded a heretic by the Solari after she found ancient artifacts of the moon. She tried to reason with the elders but was condemned for death so in self defense she killed them. The whole experience kind of made her crazy and now she vows to kill anyone who opposes the moon. No mention of any relationships.
Worshiper of a dark spider god who sacrifices innocents to maintain eternal youth. No mention of any relationships.
One of the most deadly assassins in Runeterra. She might look like an escort but there's no mention of any relationships in her lore.
Rebelled against the status quo, trading dolls for sabers. When she was forced into an arranged marriage she rebelled against that as well. To restore her house's honor she killed her father and took over their house rather than her brothers, breaking tradition yet again. Her entire life she defied stereotypes of women and became one of the most feared duelists in the world. Not at all dependent on anyone, let alone a man.
I don't even think I need to cover this one.
Inspired Ionia and came back from the brink of death to single handedly beat back a Noxian invasion. Only people mentioned in her lore are her father, her brother, and Soraka.
Street child of Zaun who became a powerful sorceress and a voice against unjust magical experimentation. No mention of any relationships.
Crazy thief who desires only mayhem and destruction. Only people mentioned in her lore are Caitlyn and Vi.
General of the ruined kind who opposed his madness to her death. Now she is a spirit of vengeance, answering the calls of the betrayed. No relationships.
An indomitable leader of Ionia bound to preserving life and peace whenever possible. No mention of any relationships.
Deadly assassin who cares only for the glory of her house. No mention of any relationships.
Immortal warrior who has fought a war against evil for over 10,000 years. No mention of any relationships.
This is starting to get really tedious but I think you see my point. Women in League are no more defined by their relationships than men. At best relationships are mentioned in passing. What is truly memorable about these characters is their strength and their struggles as well as what drives them to fight no matter what the odds.
I challenge you to show me any game or any other creative work with so many strong female characters.