Details: Quinn had a relationship in her freshman year with Dan until he slept with her older sister Taylor which ended their relationship. It can be assumed that it was a serious relationship because Taylor went out of the way to ruin her relationship and Quinn was very hurt by the action and was even still upset about it years later.
Details: Quinn and David were happily married for some time but, as Quinn would later state they stopped doing the little things for each other and everything became too routine for Quinn. This caused her to leave David and return to Tree Hill to spend time with Haley and her family, while trying to avoid David at the same time by dodging his calls. David eventually arrived wanting to speak to his wife but their talk resulted in Quinn asking for a divorce. That night Quinn made the choice to go back to Clayton's house to get David to give up hope and, despite the fact they talked all night, David did give up hope and said his final goodbye to Quinn before letting go of her for good. They left in good terms and become friends.
Details: While dealing with her marital issues, Quinn looked to Clayton for support and the two both developed an attraction to each other. Despite this, Nathan asked Clayton not to see Quinn due to all the problems she has in her life saying that she needs time to sort them out and Clayton reluctantly agreed and told Quinn they couldn't hang out anymore. Clayton withdrew from his relationship with Quinn because he didn't want to replace his late wife Sara but they eventually made their relationship official when Clayton started to move past his marriage to Sara. Clay and Quinn's relationship was made official after Quinn helped Clay get Nathan back as a client, forcing Clay to exclaim to Nathan and Haley that he planned on falling "completely and insanely in love with her". They then moved in and there relationship continued to grow closer and did fall in love. They also over came Clay being stalked by Katie Ryan and were both shot but they got through it and became a stronger couple and more in love especially following their shooting. When Clay started sleep walking it strained their relationship due to Clay being unable to talk to Quinn about it and not wanting to involve her but once he discovered he had repressed the memory of his son Quinn became more involved and eventually they became and family and quickly got engaged and then married with Logan by their sides.